On certain GB (raleigh mid-70's-type) stems, they used a whitworth nut and bolt for the handlebar clamp. I have found that M8x1.00 (fine pitch) metric bolts give something of a "class B" fit for these nuts. The difference in bolt diameter is tiny :
M8 = .31496 inches 5/16 = .3125 inches
And the 1.00 thread is very very close to the whitworth which is probably 25tpi. You may find that a similar bolt (M12x1.00 ? maybe with some sanding?) gives an acceptable fit on your seat bolt (7/16 whitworth would be M11.1125), and metric bolts are available as socket screws, i.e. with allen heads and circular vertical sides.
The only limitation is that I haven't been able to find an M8x1.00(fine) pitch stainless hex bolt ~ that's three degrees away from standard usa bolt sizing :
(5/16 x standard pitch x zinc) vs. (M8 x 1.00(fine) x stainless)
and so it is not commonly manufactured.
- Don Gillies
San Diego, CA