I've only seen Philips screws.
I seemed to remember the rivets being black only, but I just checked my only camera without film in it and they are indeed shiny!
Checking the 3003, which basically has the same, but somehow improved shutter (can reliably do 1/2000 - thought it was upgraded electronics, not the shutter itself as the 2000F can be upgraded to the 3003 "inside" AFAIK just by changing the circuitboard), I noticed that it has 2, not 4, rivets and they are flattened and black.
My 3 bodies all have 64xxxx serial numbers. Yours is a 63xxxx Maybe earlier production ones used slotted screws?
Or maybe, as was often the case, it was repaired at some point (electronics replaced) by Rollei or one of it's service centres?
Enough Rolleiflex minutae (for now)... ;-)
Anyway the seller seems serious and reasonable: If the thing is DOA, I'm sure you can reach an agreement...