If there is a spec (and no doubt the British army had one!), I have never seen it. Thinking you would pull out the old one, measure it for diameter and search. My riveting experience kind of limited - set wire in vise with "a bit" protruding - gently start tapping with the ball end of a ball and peen hammer - end of wire starts to mush out. Continue tapping until shaped as desired. Insert into position and "eye ball" how much is needed to protrude on other side. Repeat with the tapping with far end against something solid. Way more "art" than science to it. Keep flipping over to check how other rivet is forming or flattening. This also acts to pull that strap together - by continuing peening, the cross pin eventually ends up under tension, held by the riveted ends that you have formed. I have heard of it done with punches as well, to precisely place the strikes. I suspect a hollow dome end punch would help, but I have never seen one.