Anne,
the problem is perhaps more severe than you envision. I have in my possession 1 set of downtube braze-on shifters (late 1970's, engraved), and 2 sets of clamp-on shifters (one early 1970's embossed, one late 1970's engraved). To my great surprise, NONE of the wing nuts are identical between these 3 different types/eras of shifters.
I will check them tonight off-list, but I remember being shocked that I could not use my shiny-new late 1970's braze-on wing nuts on my retro clamp-on early 1970's shifters. The braze-on wing nuts were too long for the early 1970's clamps.
Moreover, the early wing nuts are "Brev Camp" and the later ones are just shiny, that's all. So I would recommend buying some clamp-ons on ebay, of the right vintage, and selling your braze-ons, to get the right type.
If you cannot get a clean set, there is a way to solve the problem. I recently had my early 1970's shifters rechromed at a local chrome shop. The chrome shop turned them into "Brev Campagno" shifters - but at least they are protected now and will not rust for quite a few years going forward.
- Don Gillies
San Diego, CA