Stainless is generally standard for exhaust flange bolts to keep corossion down. Have no idea on the size and suspect could be different for veterman vs the Vans parts. Do you have a calipar? An AN3 would be just under 3/16" on the outter edges of the threads or maybe .306" AN4 would be around .245"
Generally speaking these ball type exhaust couplings use springs and drilled shank bolts with castellated nuts and cotter keys.