[plonker]

How about when it is painted (similar to the DP labeling) on unused butt-stocks ( no holes for butt-plate screws)?

 

[plonker]

I don't believe the FR has anything to do with repair, refurb, India or Pakistan.

Maybe not, I'm not disagreeing, but what does the "FR" stand for, what's it's significance?

 

[plonker]

I think the reference is to all Mk1* rifles, not just Long Branch.
The issue being the accelerated wear of the expedient bolt release cut as opposed to the original catch.