Cavediver, the barrel flats will tell you on that late a pair if they were proofed and chambered for 2 3/4 shells. Would not be surprised if they were and suspect they are proofed for 1 1/8 ounces of shot. If so, you should feel perfectly comfortable using any modern loads up to that weight. Unless they were purpose built for heavy work (unlikely, but you will likely find 1 1/4 ounce on the flat if they were), you will find the 1 ounce target loads are a joy at the clays range.
You are being steered correctly with regard to shell pressure. In a fit of dyslexia I read your original post as 1985. If it says 2 3/4 then it almost certainly was repoofed. As others have suggested, I would keep the loads down.
