I think it's the Video Indicator. This instrument is listed as "wide-band homer" and amplifies and processes signals ? It could be removed and reinstalled when needed. I don't think that it was standard fit by 1991. By this time a V/UHF control unit was fitted...I think?
The photos below show a drawing of the Video Indicator. The 2nd is from an x gulf machine, with the empty instrument crate where the V/UHF has been removed. The last pic is the front starboard area, where (I think) the same V/UHF insrument was fitted , It is top left with 2 silver dials.
I hope this helps