I first blogged about the quad stack AK magazines (and other unique AK magazines) in THIS post.
Looks like a Quad Stack 60 round AK-74 magazine was up for sale on Gunbroker – http://www.gunbroker.com/Auction/ViewItem.aspx?Item=162814104
3 people bid it up to $900, but that was not enough to meet the reserve! It has not been re-listed since.
The company who originally makes (made?) this magazine is Izhmash. I know nothing about them, but in the Gunbroker auction, the seller says the following:
Mega rare collectible item, original Izhmash 60 round capacity magazine for AK-74.
I find it hard to believe something that was injection molded is “mega rare” and “collectible” unless it doesn’t work worth a shit and they decided to stop selling it.