The profile of these bosses appears similar to the adapter bits that Shimano uses over Campy bosses to interface with their standard downtube shifters. And the early friction versions of these Shimano shifters use a smaller d-ring screw, something like 4.5mm, as I recall. I may have some around here somewhere...
So, you could try using Shimano downtube shifters *without* those adapter bits to see if they'll interface properly. Then, you could either find some 4.5mm d-ring screws, or perhaps tap the frame bosses to standard M5 threading.
> I have those bosses on a Raleigh and a Ross. Since I am more of barcon guy
> I had a problem. If you file off the key then the boss is round and will
> not keep the adapter from spinning. So I used a dremmel tool and jewelers
> files to modify the adapter to fit over the key. Even though I was careful
> to do a neat job it's not pretty . . . but works fine.
> Howard Darr
> Kinsman OH USA