On Wednesday, July 9, 2025 at 07:38:40 AM PDT, Nick England <[email protected]> wrote:

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,

https://www.navy-radio.com/tty/28-mark.htm

I just added a section from the 1952 TT-47A manual listing differences from the TT-47.

Nick England K4NYW
www.navy-radio.com

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