most polish AK from that period had milled receivers. later guns from the late 60's thru mid 80's had stamped receivers. don't think anything made after 85 or so has been released on the surp market, and those 80's guns were cherry.
anyway, all the AK on the surp market got chopped receivers before sale to USA distributors, who blew them out as parts kits beginning in about 1997, and are still selling what little remains available. IIRC, Apex distributors got in a batch of milled polish kits about a year ago, and has been selling them since, for about $200/kit.
add a milled receiver for $300-400, US parts for less than $100, and you could build your own for $600. what you are seeing is either a basement kit build on any of a wide variety of receivers that may or may not be USA compliant, or a comercial rebuild of a kit.
are they "special"? to an AK collector, yes. to a shooter, a better deal would be had in buying a new production bulgarian milled receiver gun for similar price, or a chinese milled receiver gun for a few bux more.
