FWIW the early single screw 21" carbine barrels were factory catalog items in:
22LR
22 Hornet
222 Rem
223 Rem
7TCU
7X30 Waters
30-30 Win
357 Rem Max
35 Rem
44 MAG
They also offered a 21" .410 Smooth Bore VR then (the 45/410 carbine BBL was a 16 1/4").
The 375 Win came along in '91-'92, about when they transitioned to the 2 screw forends, so I don't remember if any of them were single screw or all two screw. I probably had most of them at one time or another all over the years, and those for the other cartridges also offered in later 2 screw mount 21" barrels. Traded the last single screw 21" BBL I had (NIB 222 Rem) about 10 years ago for a NIB Winchester Model 55 single shot 22LR semi auto rifle if that helps any. It's a pretty rare rifle NIB that I later sold a couple of years later for 4 times what I considered the 222 barrel to be worth at the time I traded it.
Probably your best bet to get a true collector value for a "set" of single screws would be to contact the TCA, or Ed. They would want to buy them most likely though, so it would be nice to know before. Maybe Ron Sable could give you an idea?
HTH
Larry