I just looked at my pile of PSA lightweight M-Lok uppers. There seems to be 2 different styles they have. Some of mine use 2 screws to tighten around the barrel nut and lock into place. Some of mine have 4 screws. The ones with 2 screws do not have the anti-rotation tabs and the ones with 4 screws do have the tabs.
For example, item 77810044 has 4 holes on the bottom edge near the barrel nut and it has the anti rotation tabs.
http://palmettostatearmory.com/psa-8-5-pl-melonite-300-aac-1-7-7-lightweight-m-lok-upper.html
Item 7790348 also has 4 holes and the anti-rotation tabs
http://palmettostatearmory.com/psa-ks47-16-7-62x39mm-moe-13-m-lok-lightweight-rifle-kit-7790348.html
Item 7789433 has 2 holes at that bottom edge and has no anti-rotation tabs.
http://palmettostatearmory.com/psa-10-5-9mm-12-slanted-m-lok-railed-upper-516444990.html