I've got an hr bates bar frame with braze on boss for downtube shifters. I have some campag shifter but any I have Googled have a lever stopper that looks like it needs a square section to fit over which is part of the braze on boss. The one I have doesn't have that. Any ideas? Quite stumped at the moment.
I'm not an expert on the history of downtube shifter bosses, but what's going on here may be the bosses on this frame were made to emulate the boss part of certain Simplex band clamp type downtube shifters. If you google around, you can see various evidence of Bates often speccing Simplex.
If the dimensions/threading of the bosses were enough like a standard modern one in every other way, you could look at getting some downtube shifter bosses from a framebuilding supplier and simply cutting off the post part, leaving you with just the square, getting the hole just right, then taking the paint off the frame in just that area and epoxying or brazing them on.
It's also possible they're meant to be used with some shifter or another that simply doesn't have a stop.
I believe the shifter boss is made for Huret shifters. This link may help: https://www.disraeligears.co.uk/site/huret_accessoires_cycles_cyclomoteurs_motos_-_1975_page_13.html