It looks like you've been very busy!
I notice a couple of intriguing wire links on the underside of the main board.
Re: sensors (and chip numbers); yes, it seems that Timeter were very keen to guard their secrets.
I am also suspecting that a daughter board formed part of the GPIB "package".
On the face of it I disagree with you about J2 being for RS-232, although I don't have the time for any more "research" just now - except to mention that the Signetics 74LS154 (U8) is (was) a "1-of-16" Decoder/Multiplexer (datasheets are available on line).
