Yeah, you have to add them to the cart to even see the part number. They look the same because they used the same exact photo...
It appears to be the semi auto upper, used in both pictures since you can't see a sear cutout.
You would think the SMG/MAC-10 version would specify open bolt. The fact is says Mac10/MPA seems a bit misleading, along with the description. "Ready to be dropped into your Mac or MPA".
Masterpiece Arms never manufactured an open bolt Mac style weapon. They weren't founded until 2000.
@Beowulf , what was the consensus from USMG? I hate to make assumptions but, since they came here to point out you bought the wrong one, I can only assume.
Here's where you can find the part numbers their cart...