I believe those mags are mentioned in "The Complete AR-15 Assembly Guide", or whatever the heck is the name of that book.  

I'll check when I get home.

I'm searching my books now.

I was wrong, it was NOT in one of my books.  I did find this from Troy's Mag FAQ:


[3-2-2-4] Thermold 30/45
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Among the most unique magazine designs, the 30/45 magazines were designed with an extendable bottom section that would allow the magazine to be fully loaded to 30 rounds and stored (with the base extended) with the spring under minimal pressure. This allowed long-term storage of fully-loaded mags with no wear to the spring or the feed lips. They can also be loaded with 45 rounds when the base extended, though they shouldn't be stored in this configuration.


I could have sworn I've read something more about these though.  I'll keep looking.

Until the little locking tabs break off, that is.  They were originally designed to be one-time-use mags that could be stored pre-loaded, but with the bottom extended to reduce spring pressure.  When you were ready to use the mags, you would collapse the bottom into the mag to tension the spring.  But the tabs that hold the bottom portion weren't really designed to be unlocked very often, and will break off pretty quickly.  Once that happens, you have to duct tape the bottom portion closed to have a usable mag.

In other words, there's a reason they were never popular, and they certainly aren't worth the "collector" prices being asked for them, unless you're a collector.

-Troy