I'm following up to my own post here. I did a bit more investigation, and found that the internal thread diameter of the above braze-on is about 3.8 mm; also, a #10-32 screw (4.6 mm major thread dimension) will not thread in at all (too large in diameter, by just a bit). By doing some reading on the UK
Classic Lightweights site, I came up with this, in their
Hints and Tips article #1:
"
2BA bolts - Looking for replacement bolts for the drilled and threaded mudguard eyes found on most 50s frames? I had the threads checked and was told that they are 2BA which seems right and I have been able to get some from a local tool shop in polished zinc finish with cheese heads at about £1.50 for ten with nuts. The shortest I can get are 1” so some sawing is necessary. It seems that they had some brass ones until recently but the stock will not be replaced - they say they cannot get them. I guess many owners take the easy way to tap out and fit 5mm bolts."
Well, it turns out that this (to us, obscure) thread standard is also used most commonly on the threads holding the metal pointy part onto dart (pub game) shafts, and on British carburetors, but the reference to mudguard eyes (nowadays 5 mm x 0.8,
which is also used for shifter screws) got me looking in that direction. I find on
this site that the 2BA thread spec has an internal thread diameter that fits with my measurement:
Accordingly, I've found some stainless round-head slotted screws on
BritishFasteners.com and ordered 4 of them -- at $5 a pop with postage and taxes. I will report back here when I've had a chance to try them out.
Yes, I could just retap the braze-ons to 5 x 0.8 mm, since I will never install Cyclo shifters, and probably nobody else will either. But I don't have a "bottoming tap" for that, and am loath to drill through the downtube just to retap. I have two different frames that have these braze-ons, so if I've found a workaround, that's good.
By the way, my original post above does not make the point that, while these braze-ons
will take the square backing/stop plate for modern shifters (12 mm nominal square hole/boss) and they have a round boss exactly 9 mm in diameter that projects 6 mm exactly from the square, they do not have the "Campy-standard" flats ground onto part of the round boss. A shifter like the Simplex Retrofriction doesn't care about that, but others use the flats to key parts of the shifter that should not rotate in use.
And, in case you're wondering what I'm blathering about,
here's a grainy image of the type of shifter that these braze-ons were intended to mount, along with the braze-on in question itself:
The braze-on is L.25A (1 shilling, 1 pence) and the friction screw is SP.58 (only 2 p!).