I don't recall seeing this best I can remember.
A current auction has a S'G' Flat bolt marked S'G' on the end of the right lug, like we normally see.
But under the right lug a number 2 can be seen. I suspect they might be worn off numbers that were in front of the 2.
See pictures......... Any thoughts ?
https://www.milsurps.com/images/impo...5/sl1600-1.jpg
https://www.milsurps.com/images/impo...5/sl1600-2.jpg