SAI - Springfield Armory, Inc.
It's a 3rd-Gen.
I also have an un-marked almost identical unit which has a steel allen screw that bears in the vertical receiver groove--I like THAT much better than the all-aluminum SAI. The rear screw, going into the replacement for the s-clip guide, is about 1/4-inch further forward on the SAI, eliminating the ability to just drill and tap the guide (like I did...). I HATE that about the SAI.
The SAI, on the other hand, does have a round groove in the bottom usable for shooting with the iron sights out to about 300 or so. THAT I like!
The un-marked one uses captive thumbscrews. That's much better than the SAI "lose them" non-system. The two grooves on the Weaver "base" are NOT Mil-1913 spaced. Hate that. The SAI is Mil-1913. I'm thinking about having Pictatinny grooves and the sighting tunnel cut into my other one.
Now you have more data points than you want...
You can often get used SAIs for $75 or so. Just really check out the vertical rib, 'cause improperly mounting them can make them shear off.