Yeah then it will be some jeffed up sort of UNC.
Even if you did ID them you?d pay too much. Also, I would bet modern tensile bolts are much better and much stronger tbh.
Modern nyloc nuts are much better too
Problem with the sceptic tanks is like us they have different TPI or threads per inch. You have UNC (coarse) and UNF (fine) and we have 1.25, 1.5 and I'm rare cases 1mm.
The bolt situation is real, and they always use the hardest to find so you go to them and never make the info public either.
Which is why I am now a walking bolt detector.
It's even worse when they stop making spares and just bugger off, leaving you with something that cost a fortune.
My pal, god bless him, bought me a kit designed for finding the correct threads. So I'll be using those I'm sure of it.