The initial goal for a "perfect" typewriter is to offer the ability to use both Selectric Composer elements and typewriter elements. In addition, elements for the Mag Card Executive should be usable as well.
This goal is relatively simple to achieve.
Start with the Electronic Composer. Add a fourth series of openings to admit light to the wheel governing horizontal motion so that there is a 1/60" escapement, in addition to the three Composer escapements of 1/72" (red), 1/84" (yellow), and 1/96" (blue).
Next to the lever choosing between the three escapements, also include another lever like the one on the Selectric II typewriter for choosing between 10 pitch and 12 pitch. But give it two additional positions: one with a circle, for Mag Card Executive elements (or, rather, for 88-character elements that look like the elements for the Mag Card Executive, but which use the characters as slightly modified for the Electronic Typewriter 50 instead, as it appears the Mag Card Executive did in fact use a 1/72" escapement; that is not to say that the typewriter, with some improvments to be described below, could not handle those elements, but the settings required would not match what was originally printed on their caps) and one with a triangle, somehow differentiated from the one for 10 pitch, to indicate that a Composer element is being used.
Thus, such a machine would have both an Escapement Lever, like the Selectric Composer, and a Pitch Selection Lever, like the Electronic Typewriter. The diagram below
illustrates one way they could look, with the Escapement Lever on the left.
Fools Rush In Where Wise Men Fear To Tread
There are four complicating factors that arise when trying to combine the Selectric typewriter and the Selectric Composer in a single machine.
Thus, there are several ways in which the Selectric Composer behaved differently from a typewriter.
Such a machine would clearly need a Leading Dial, similar to the one on the Selectric Composer, by means of which the line spacing could be specified in units of one point. If, in addition, it had a Line Spacing Lever, like a typewriter, by which single, one and a half, and double spacing is chosen, then an additional lever, which selected which of those two means of controlling line spacing is used would be needed, and the setting of this lever could also determine how the platen knobs behave.
The arrangement of characters on the element could be decided by the setting of the Escapement Lever. It is preferable to use this lever, instead of the Pitch Selection Lever, since there are various circumstances under which an element might be used with a different setting of the Pitch Selection Lever than is normally used for it, but using a different Escapement Lever setting is almost never helpful. One way to deal with this would be to allow the default behavior to be overridden with Code key combinations, since it is clearly essential that the right arrangement of characters is used, even if it is desired to use nonstandard spacing with an element.
When it comes to whether tab stops should be settable anywhere based on the Escapement Lever setting in current use and the setting of the Pitch Selection Lever, or only at intervals of 1/6 of an inch, a pica, which are shared between all settings of the Escapement Lever, while using the Escapement Lever to choose between typewriter and Composer behavior might work nearly all the time, there are situations where this might be awkward. Another option for this would be to use the lever that chooses between the Leading Dial and the Line Spacing Lever to control this, since it indicates the user's intention to use the device as a Composer or a typewriter.
An option I had considered was to put a slider next to the Escapement Lever which could be positioned so that the dividing line between the wider escapements for which tab stops keyed to current settings would be used, and the narrower escapements for which the shared tab stops of the Composer were used could be placed by the user. That way, it could be set so that setting the Escapement Lever would usually take care of this setting as well, and it could be moved when a non-default behavior was desired.
Although the Electronic Typewriter models 50, 65, 85, and 95 also offered proportional spacing on a Selectric element, they did not raise these issues.
The proportional spacing system was limited to a lower maximum width on the Electronic Typewriter, for which the issue didn't arise, as the new 96-character element was designed from scratch.
These proportional spacing typestyles were all about the same size, so no additional line spacing flexibility beyond that of a typewriter was needed, although, as this size was somewhat large, the Mag Card Executive normally came with a ratchet wheel which gave a single spaced line higher than 1/6"; previously, I discussed the issues this led to which prevented a version of the Symbol Proportional element being made available for the Electronic Typewriter.
And the proportionally-spaced typestyles for the IBM Electronic Typewriter were built around the 1/60" unit which could also be used as a common basis for 10 pitch and 12 pitch spacing. So there was no reason not to use the 12 pitch tab stops for proportionally-spaced typing as well.
Note also that while the first of the four issues identified for combining the Selectric typewriter with the Selectric Composer would not apply to adding Selectric Composer typefaces and capabilities to the Daisywriter typewriter, the other three issues still remain. On the other hand, there was no fully satisfactory resolutionfor to the first issue, while measures to address the next two at least approached adequacy, and the final issue of tab stops could only be resolved by including a means of directly selecting the desired behavior.
In addition, the fact that the Mag Card Executive apparently had a 1/72" escapement adds an additional source of confusion in the use of a "universal" typewriter.
There is one other respect in which I had thought, but mistakenly, that the Selectric typewriter did not achieve parity with some designs that preceded it.
The Blickensderfer Oriental was a version of the Blickensderfer interchangeable element typewriter that could switch over from typing from left to right to typing from right to left. I had been aware that there were Selectric typewriter elements for Arabic and Hebrew, but I had only seen pictures of the Arabic-language version of the typewriter, which could only type from right to left.
In fact, however, IBM also made a Hebrew-language version of the Selectric typewriter, and this one did include a lever - in the position normally used for the index key - which changed the typewriter's direction of printing.