Yeah, I never got it either and thought the whole magpie thing was retarded... I've seen a few of these forum fads come and go here over the years.
The Automag rail will use the Automag sear.
The RTPro will use the RTPro sear.
Salad Fingers on MCB just listed like 6 of them NOS for sale the other day. I think his price of $80 is a little high, but does include a sear, so not