I just came across an old Am Std toilet which has the original flush valve using the tilt disc but has been fitted with an adapter which enables use of a regular 2 in flapper in place of the tilt disc. Is this adapter available anywhere?

It is probably something like a Fluidmaster glue on adapter. If so they are a bit undersized so you have to squeeze the adhesive putty out to make it reach the rim of the flush valve.

I wish I had taken a picture.

I think you are referring to the Fluidmaster flush valve seat renewal kit. This was nothing like that. This was a small plastic fitting that mounted on to the two holes that the barbs on the tilt disc fitted on to and had two ears that hold a flapper. I used a regular Korky flapper and it fitted the flush valve seat just fine.

Korky flappers work well for me, the Fluidmaster replacement discs for the tilt disc mechanism not so much. The FM discs tend to "chunk' out and develop little craters after about a year of submersion in Toronto water.