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.
I've worked on rifles with the SAI Gen 3, the Sadlak, the Smith, the crappy $60 ebay copy, and the ARMS. Although it requires that you hold your tongue just right sometimes, the bolt will come out with any of these setups.