The mark is lightly struck, or was partially removed as part of the barrel final fit and finish Kevin.
Another possibility is 'P' - maybe Sanderson‘s Pittsburgh Works?
Our only hope would be to find original Hunter Arms, Fox, Ithaca, N.R. Davis or Lefever purchase orders for rough forged tubes from Crucible or Sanderson Bros. - all have barrels with the SB&Co stamp. We have no idea why the T, G, F & P were added.
We can say with confidence that Sanderson/Crucible did NOT make the pattern welded or fluid steel tubes.
BTW Lefever damascus patterns are discussed (along with those of Colt) here
https://docs.google.com/document/d/1...g/edit?tab=t.0