FWIW - The earliest 28KSR machines (Mark I) were delivered to the US Navy as TT-47/UG. They were quickly replaced by the Mark II model TT-47A/UG. I have never seen a Mark I Model 28 except in the 1950 manual.
Some differences between Mark I, Mark II, and Mark III are illustrated here,
I just added a section from the 1952 TT-47A manual listing differences from the TT-47.