Clive,
Don't let the fact, that someone bought the other Flues O/U for 25-50% of what it should have brought, get to you...That guy didn't advertise the auction, he didn't list the item correctly, he didn't use the name (word) kornbrath in the electronic auction listing.... he didn't so much as Google the name Emil Flues, or he would have had a better handle on what he had....
As for being handmade....the young men who stood on the elevator car (while on Seneca St.) to watch him work, told me many years later, that the recievers where inletted with a wall mounted crank drill, die sinkers chisels, breast drills, files and most importantly lots of sunlight...
If this is so, it should be evident if ever the occasion arises to disassemble the gun. Meaning there will be no mill marks (swirls associated with milling machines)
Did you get any history with the gun? It's a well known fact that Emil and Rudolph traded work...that is; guns for engraving...but we don't know what guns represented Kornbraths payment....does your gun have a solid rib?
also, as Larry said, square crossbolts are not uncommon...but those fangs on the rib extensions are