I received my Matrix Arms receiver set today from the Primary Arms Black Friday sale - it looks very nice for the money spent.
It has a threaded bolt catch , wish I knew that when I ordered since Primary Arms lists part no. LR-12-308 for $ 3.99
Now if you try to order just the pin it is plus shipping and over $ 10.00
I hate pissing money away so I'll probably just make one.....
EDIT: stopped by ACE Hardware on the way home , a 6-32 socket set screw was too big.
This lower takes a 4-40 size threaded bolt catch screw!
You can probably just drive in the standard roll pin and be done with it.
I'm not sure anybody even sells a 4-40 bolt catch screw....