In the thumbnail milling mark it looks to have the remnant or start of the groove found on SG frt sights. But the number on the side makes me think S'G'. Might have been made during the change over at Grand Rapids. Then a parts transfer to Rock Ola. It might just be me but it almost looks like both on the left and right even with the rear of the sighting blade like there may be a number or letter on each side. Above the arc of the thumbnail....like a 7 on the left flat and maybe a Y or N.

Your coming out of the woodwork with some interesting stuff lately Jim.
Here's a picture of a N marked frt sight on my S'G' 1,823,135 with the thumbnail milling mark. There is no number on the outside ear. This would fall into IP's first serial number block. Weird how your's shows signs of being a possible SG, S'G', or IP but ended up being a Rock Ola marked one.



Here's a couple pictures of a IPN marked frt sight which shows some milling on the left side of the rear blade and the number on the outside. The side profile looks very much like the RP one in your picture,

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