Yes, the metal extractor port guard is one of the minor changes from the 550A. When they went to a single extractor, the cases would hit you in the head without the guard. My old 550A has the dual extractors and throws the brass smartly to the side (no guard).
You got a smokin' deal! I wish mine was grooved for a scope. With open sights I can't shoot very well anymore. I can drill and tap it but I hate to modify a perfectly good gun. Years ago, I shot many critters and lots of beer cans. It is supposed to shoot shorts, longs, and long rifle ammo interchangably. Mine never did work well with shorts but it operates nearly flawless with any brand of LR ammo.
If you look at the barrel, just in front of the receiver on the left side, you will see 2 or 3 letters that are the date code for when it was built. Mine is KRR, which indicates May of 1946, the last year of production for the 550A.