In the vein of, "I don't know everything, but I know people who do," herewith Curt Arndt's reply to your inquiry #2, above:
The hex bolt heads with the depressed circle are proper British bolts/screws and they denote that the fastener is the newer Unified Standard (UNF/UNC) versus the old Whitworth Standard (BSF/BSW) which was slowly phased out in the 1950s. This is why BN1/2s have half Unified fasteners and half Whitworth fasteners, i.e., engine, gearbox, and suspension..
Unified fasteners are the same as SAE fine and coarse or ANF/ANC (American National Fine/Coarse). These are the fasteners that you can find in any modern hardware store. The "triangle" on the head is just one of many markings on US-branded hardware, and while the proper size and thread pitch, and they will work, THEY ARE NOT CORRECT for [Austin-Healey Concours Registry] concours.
[Note in Austin-Healey Concours Registry inspections there is a half point deduction] ... for EVERY incorrect fastener with NO upper limit. Therefore, [if you have] 100 incorrect fasteners, you start out with a Silver award, losing 50 points [out of the theoretical perfect car with 1,000 points] right off the top.
For what it's worth, besides the depressed circle, there may be a vendor name (WILEY, WODEN, TWL, LINREAD, AUTO and about 35 more) and a letter code (B, R, S, T, V etc...) denoting the strength rating.
Curt also sent several dissertations of post-doctoral quality on the subject. They are in PDF format and guaranteed to make your eyes bleed in their level of detail. Send me your email address in a PM and I'll forward them to you, but only if you're a true glutton for this stuff.