That's a "light barrier"/photointerruptor.
Armed with dimensional and supply voltage knowledge, and deriving LED current from that (by guestimating a 2V forward voltage and doing the series resistor math), you could probably find compatible replacement parts on one of the large distributor websites.
Considering this board's component selection looks like the 1980s, it's unlikely that the original part is still on stock anywhere: it's not high-volume / cheap enough a part that it got produced in the billions, and it's not valuable / mission-critical enough for defense-specific electronic stockholders to have it.
By the way, I'd start with unsoldering every single one of these glass diodes very carefully and checking whether they still act as diodes – I'd expect them to be more susceptible to failure than the photodiode in the photointerruptor.
Whatever you do, start with drawing and tidying up a full schematic of this board – pretty much impossible to get the right polarities and voltages if you don't have a clear idea of how the photointerruptor integrates into that part.