Unconventional bolt - please help identify

[Tidybuoy]

looks identical to the bolts that came with a toilet wax ring I just installed. You could probably find these bolts at a hardware plumbing section

[FPB111]

Trailex also uses them to hold most of their aluminum trailers together. Drill a hole in your anvil and hammer a hex bolt head flat then file/grind to fit.
Also
https://www.rockler.com/t-slot-bolts-pack-of-five-1-4-20-thread?country=US&sid=V91040&promo=shopping&utm_source=google&utm_medium=cpc&utm_term=&utm_content=pla&utm_campaign=PL&tid=pla&gclid=Cj0KCQjwgo_5BRDuARIsADDEntSo-BjBVN3dzt6yPHelx48C3sl6QhHk6XwnOfodQRPO-QIYilrM05EaAks_EALw_wcB

[Bob Kontak]

Originally Posted by 1990C4S

Those look like resistance weld studs to me, they should maybe have be projection welded into the door about 100 years ago....

There are absolutely weld marks where the original weld studs were positioned.

However, the only stainless acorn nuts were 1/4-20 at Lowes, so brass toilet bolts for the win.

[Tidybuoy]

Originally Posted by Bob Kontak

so brass toilet bolts for the win.

New, I hope

[aschen]

I love me some home depot engineering.

[Scott Douglas]

Dad's old '64 Travelall had mirrors a lot like that on it.
Had three gas tanks @60 gals total, split rims, it was a 1200 series.
They don't make them like that any more.

[notmytarga]

Beat me to it, by a month - toilet bolts is what I thought as well.

Those non-wax rings are great. Reusable and a more secure seal. The void in the base of the toilet makes a good hidey hole. These rings make it simpler to use it.

[Nostril Cheese]

T bolt or Trim bolt.