Mystery (to me anyway) Model 28D Typing Unit

The mystery is a Model 28D typing unit - it has a different gear to drive the keyboard - is slightly larger and has more teeth that the usual typing unit - it will not fit into my 28KSR or my 28ASR. Why drive the keyboard with a different gear? What was it used for?

This 28D typing unit also has some extra mechanism on the right-hand drum - looks like blades that trip a switch contact located near the main shaft. Is this some sort of tab mechanism?

p1010001.jpg (148343 bytes)
28A typing unit -
keyboard drive gear in center
- 21 teeth
p1010006.jpg (135340 bytes)
28A typing unit
no extra switch contacts
p1010004.jpg (152839 bytes)
28A typing unit
no extra mechanism
 
p1010002.jpg (127628 bytes)
28D typing unit -
keyboard drive gear in center
26 teeth, 0.1" dia larger, tighter pitch
p1010005.jpg (190387 bytes)
28D typing unit
extra switch contacts in upper right
p1010003.jpg (157339 bytes)
28D typing unit
extra mechanism on right-hand drum