Wow. Equally glad I stocked up. Bought a load of Remington #10 and 11 from some gun shop in north Cambridgeshire I stumbled across back in about 2017...
Glad you got it sorted and offers still open to anyone if I find that form, but currently as long as its properly declared it should get through even...
If i was in your situation i would, if i have a lathe bore out the 28 tpi thread and press fit a tight insert treaded for 20 tpi, but i would have to...