[GreenKeys] TT-168/FGC-39 TD - what is it?

[Nick England]

John K4OZY and I opened up a mystery box today to find a brand new
TT-168/FGC-39 TD.
Still sealed in original military packaging and seeing the light of day for
the first time since 1956. Great fun, photos coming later.

This is an MXD-28 "multiple transmitter distributor" unit with a Navy
contract number.
So here's the question -  what's an FGC-39 "teletypewriter set" that this
thing was used in? Does anyone have a photo or pointer to more info?
Maybe it was an 3-unit MXD (3 tape readers on a single motor shaft)? - was
it one of those pull-out drawer units? - Where have I seen a photo of that?

cheers,
Nick KD4CPL