Anyone seen this on back of Receiver?

[USGI]

I've got just one Inland with an SG marked receiver. Had it almost 9 years now and never did anything with it other than pull the action out of the Winchester stock briefly. Wanted to check on the TH group and OP slide to see if they looked "right" - it's pretty easy to get to, so maybe tomorrow will pull it out of the safe and see if it's got a dimple at the back. I doubt it will, or the former owner would have posted about it here on the forum. - Bob