Not much info I can recall, just some dim memories of the old ads and catalogs. As I recall there were at least three similar versions of a "dress up kit" that al utilized a pistol grip extending stock and bbl shroud. One was a dual wire affair, think M3 greasegun with a plastic buttpad instead of all metal, it was chrome plated. The other two looked like the one in the OP, but the more common one had a latch on the bottom and used holes in the inner tube to hold the adjustment like an AR15 style carbine stock instead of a collet. RamLine made an extended stick mag and Mitchell had their belt driven teardrop drum for these and 10/22s. I believe Parallax was the main seller of the wire type, IIRC they usually had a 2 page ad inside SOF, SWAT, etc back in the 80s and the AR7 stuff was at least a quarter page. Even sold a "dragonhide" pouch for the Mitchell drum, looked like regular cow skin to me.
Found an ad for the collet one in the box, Morgan Arms of Las Vegas, never heard of them. FedOrd either made a similar one or just sold these, I still recall seeing them in their catalog or ad..
https://www.gunauction.com/buy/10350903
This is the CAR15 style one:
https://www.ar-7.com/AR-7AdjustableStock.html
This is the wire slider, it's marked Mitchell Arms.
https://www.gunauction.com/buy/10493049