I found one on ebay that is novel to me. Someone has placed a finished lower in a container of resin in order to leave a deep impression. Where holes should be drilled in the lower, sleeves appear to be in the resin. I assume one would align the drill-press with the sleeve, place the unfinished receiver in the impression and then drill.
I think it would be better if the sleeves were instead transfer punches to leave a dimple on the unfinished receiver where the hole should be drilled. You would have to have one mold for each side of the receiver.
Alternately, you could use the sleeves but have two molds that will totally encase and immobilize the receiver while you drill each side separately.
It looks to me as if this would be easier, more accurate and beginner friendly than the expensive jigs.
The exterior dimensions of the lowers would have to identical, or all the locations could be off. In theory, not a bad idea, if all the lowers came from the same manufacturer.