IIRC the 'R' is a mark that indicates the gun was recv'd into the Ithaca Repair Shop.
It doesn't tell you anything about what the problem may have been nor any repair done there.
I think it was stamped just as the gun was recv'd and even before it was passed along to any of the Service Dept 'smiths.

Seems Walt Snyder posted that info on older threads when the same question came up.