First off that is a beautiful G-1 kit! Looks like it was re-parked and the wood is in great condition. Yes I believe most metric mags will work though someone else might have had other experiences.
The "locking lug" is most likely the "locking shoulder" seen here Locking Shoulder, .256 - .267-D S Arms
this is the way Fals are head spaced.
As far as which reciever; I perfer DSA receivers as they just look nicer. But at the end of the day both receivers work it is just price/looks.
If you dropped 9 bills and that included the receiver then that's not bad, if it was without that receiver in the picture then ya it wasn't such a good buy, but you have rare and a beautiful kit!