OK if you mean the setscrews which clamp the two halves of the metal bow curved sections together, they are 5/16 UNC. Unfortunately I have only just enough for a full setof bows which I will need in the not too distant future, so I'd like to hang on to them (well actually I'm 3 short!).
On every bow I have - new or old, the thread is just a little bit tight, but after a few turns soon loosens up. Thread profile is just a smidgeon out on outer diameter I think. I tried 5/16 Whit but worse. Being U.S made they would have been UNC for sure.
I decided on stainless steel but can't remember after only 12 months if this was because the only decent head profile was available in s/s (don't want to cut the canvas :cool2:). Anyway they are a socket head dome profile.
If you look at a new pair of bows you'll choose 1 1/2" length like I did, but when you've assembled them all on the truck you may decide you'd have been better off with 1 1/4" length as they all stick out 1/4"inside when they've squeezed together around shrunken wood.......may need the odd longer one or a clamp and it is n absolute sod getting the holes lined up to get the thread started (but hey that's GMC ownership for you;)).
Also I can't find the invoice (pity 'cos they were good value - I seem to think it was A-1 Stainless or something?), but there are loads of s/s fastener stockists out there on the web, so you'll need to hunt around to get sensible price. This is a typical one from ebay and shows the head profile, not a bad price either for 20 pack (also 3 pack available):
http://www.ebay.co.uk/itm/Stainless-UNC-Button-Head-Bolts-5-16-x-1-1-2-20-Pack-/370244274575?pt=UK_Motorcycle_Parts&hash=item563447498f#ht_2255wt_1159