The Marshal Tool and Supply Corp wants over $65.oo and MSC Industrial Supply Company wants $100.oo for that "Special tap". Ouch !!!!
I'd bore and single-point thread that AR frame in a lathe. A 4-jaw chuck should hold it (backwards) by the buffer tube fitting. Use a long reach drill bit to start the pilot hole then bore it to proper minor thread diameter with a boring bar... a long and rigid boring bar. Then single point that aluminum thing with a long reach threading bar being careful not to allow the rotating receiver front to hit the saddle or toolpost all the while.
This assumes a lathe and single point threading skills.
One other method is to use a lathe to make a special tap in a decent piece of steel. Perhaps a piece of old rifle barrel can provide the steel. I doubt that any heat treat would be needed for one or two uses in 7075T6 aluminum. I'd still bore that minor diameter hole on a lathe as above or with a micrometer boring head in a mill.