[GreenKeys] Model 28D typing unit question

[Nick England]

I am new to the mysterious ways of the Teletype and seeking
enlightenment ----

I have several Model 28 typing units on hand as I am in the process of
restoring a 28ASR.
One typing unit says "28D" and has differences that I am curious about
a) it has a different keyboard driving gear that won't mesh with my 28KSR or
28ASR keyboard drive. Why this larger gear with more teeth?
b) On the right-hand (spacing) drum it has some tined keying mechanism (like
a security-light timer) that actuates switch contacts - Whazzat? I guess
some tab mechanism since the tines occur every 7 spaces?

photos at http://www.virhistory.com/ham/rtty-05.htm

cheers and thanks for all the great help you guys provide,
Nick KD4CPL
http://www.virhistory.com/ham/rtty.htm
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