I've been torture testing an old 20rd mag of mine that I've fitted with an H&K follower from 'the good' H&K mags. It has survived and flawlessly fed on semi-, rapid-, and full auto in Albuquerque's 115° July heat to Valhalla Training Center's micro-blizzards and everywhere in between. So far so good...

There isn't any real information on these so I googled the Patent Number and came up with these results:
PAT.NO 4139959

United States Patent 4413437

Abstract: An improved cartridge magazine constructed of high impact nylon resin and in which a spring urged follower pushes cartridges into a rifle into which the casing is inserted. The follower is attached to the spring which prevents fore and aft movement of the follower and the spring is curved to physically contact the bottommost portion of the follower to prevent the follower from pivoting about the spring contact. A removable floor plate is removably attached to the casing by means of a pair of resilient latches. The floor plate is symmetrical and has no preferred orientation, thereby simplifying loading of the cartridges.


Inventor Howard; William J. (P.O. Box 573, Wilson, NC 27893); Harvey; William A. (P.O. Box 3065, Wilson, NC 27893)
Owner/Assignee
Publication Date February 20, 1979
Application Number 796560
Filing Date May 13, 1977
US Classification 42/50
Int'l Classification F41C 025/02
Examiner Jordan; Charles T.

Really these are The Thermold ones look for "Thermold D&D INC WILSON NC" on the left side. "PAT NO. 4139959" is on the bottom


I hope this helps.

I did some additional research as I was looking to buy some more of these 30 round mags...and as it turns out they no longer make them because the company went out of business:

Here is the link where you can read about them. Aparently they sold for $25 each at some point prior to the company going out of business:

http://www.google.com/search?hl=en&l...mold+D%26D+M16

I've got one of the Thermold 30/40 rd mags (Patent # 4472900). (Push a detent and the mag bottom extends to hold 10 more rds.) It was given to me back when we were experimenting with "good idea" gadgets. We never purchased more because it didn't solve any pressing need and you couldn't fit three of them in a standard ammo pouch. Personally I never had a problem with it, just never saw the point in spending money on aftermarket mags when I have/could get GI mags "inexpensively". I keep it mostly as a novelty item though it does allow me to keep 30 rds loaded in the mag with the bottom extended to relieve the spring pressure. To use it I just slap up on the bottom of the mag to close it and restore full (actually extra) spring pressure. FWIW - Peregrino

Surefire 60/100 rd. Rifle Mags (Early April Fool's Joke?)
 

http://militarytimes.com/blogs/gears...fle-magazines/

Would not have given this a second look if they didn't happen to be made by Surefire. :munchin

Magpul is also working on their own version:

http://www.defensereview.com/magpul-...-ar-ar-15-mag/

loaded magazine
 

Considering pressure on the magazine spring, is there a rule on the length of time a loaded magazine should remain loaded or cycled in storage and still GTG. Thx in advance...

Only time having too many is detrimental is when you have to swim with them. If not, buy when and how many you can afford.


CD

What kills a spring is repeated compressing and uncompressing (so I've been told). I have mags that have been loaded for years....to no ill effect.

TS, warm regards this New Year, thx on the mag question.

Concur on the spring issue. I’ve had mags loaded for close to since time began and they still worked. Last time my Benneli Super 90 got fired no problem after having seven shells stuffed in the tube for well since it was invented.

As far as detachable mags go I think I have all could ever need but maybe I’ll buy ten more if the price is right. :D