If you entered the model T in a restored car event then you would loose brownie points for using Nyloc nuts.
Although they work fine, other competing restorers might go to painstaking efforts to use correct to the era nuts. If its a museum piece, or may be one day, that's valid reasoning. If the owner wishes to take his Brunswick restoration to such levels I guess it's his call where he wishes to draw the lines. Many may see it as unimportant, as they are taking the perspective of functionality and not true restoration.
I have a early Brunswick Balke Collender , maybe 1915 or so, the bolts for the pocket irons were missing and a unusual but not impossible thread between fine and coarse. Also square heads. I helicoiled the pocket irons to take modern bolts rather than seeking out or making the correct ones.
I had one slate bolt missing. I left it alone, maybe I'll make or find one. I stuck an expanding type of fastener in there for now and cant' tell which rail has the missing one.
its a special bolt with threads in the center, and some BBC's attach an ornate bit there, on mine they are hidden by wooden rails that cover the bolts.
I could see revisiting that one day but it's not important or time sensitive and I dont really care so much that two pocket iron bolts have hex heads.
I dont think it would be wrong to fuss over such details, just a decision not to.. I gave it thought and came to the conclusion that I can choose to just play pool and be happy enough with it all instead. ;-)