Charlie, I've been watching some of those same type trigger housings on eBay and GB. A guy sold (2) of them recently on eBay and one of them was a Type 3 with a narrow mag wall. If I remember right, after the takeover, the tooling hole gradually changed to a dimple and then disappeared. Either that or the hole disappeared gradually and changed into a dimple? Take a look at these listings and the differences in them.
http://www.ebay.com/itm/130631594478...84.m1438.l2649
http://www.ebay.com/itm/130631521306...84.m1438.l2649
That S'G' marking (in a box) on the one you linked to - I've seen that before and wondered what the deal was. Also notice on that one that the milling on the sides leading up to the mag wall has the circle shape where all the others don't.
Paul has one of the earlier trigger housings listed on GB right now. It's a good example of the features I think you are questioning on the 'M1partsman' listing. I wonder why Paul's has the straight hammer mark, though?
http://www.gunbroker.com/Auction/Vie...Item=269804872
All this makes me wonder if there were others making some of these trigger housings and supplying them to Grand Rapids?
Here's a couple pictures of my S'G' Type 3 housing with the Type 2A hammer. - Bob