I had some old Muzzle Blasts magazines out, for the years 1973 and 1989. I didn't find any Green Mountain ads in the 1973 issues, and ads in the 1989 issues only list calibers and outside dimensions, with nothing about rate of twist. However, the one printed Track of the Wolf catalog I have on hand is #16, dated 2004. This shows a twist of 1 turn in 48 inches for .32, .36, and .40 calibers, 1 turn in 60 inches for .45 caliber, and 1 turn in 70 inches for .50, .54, and .58 calibers, including the Green Mountain "drop in" replacement barrels for T/C. An additional note in the catalog indicates "Grooves cut .010" to .012" deep, depending on caliber," and "Each barrel has eight lands and grooves."
I don't know when Green Mountain started making round-ball muzzleloading barrels, but I think it was some time in the mid to late seventies. I can't say for sure, but I think their rifling specs have remained consistent. I know the old Douglas barrels were generally 1 turn in 66", and I have a Fox Ridge Outfitters catalog for 1999 that shows the round-ball replacement barrels made by T/C for their own rifles had a 1:66 twist, also. I'm wondering if Old Hawkeye's barrel assembly might have been made by T/C rather than Green Mountain.
I am reasonably confident that your .50 caliber Green Mountain NOS barrel would have a 1:70 twist. I would expect to find that, if I were to buy it.
Best regards,
Notchy Bob