They were prototype mags. They are curved to allow the taper of the cartridge to set the curve of the mag. They fed great. They didn't pass the cold/drop test at all.
They tinkered with it a while longer, then it was basically forgotten till the 25mm XM109 was introduced. The 25mm shell fit the polymer mag and fed good. Don't know if it got any further.

Go to biggerhammer.net
There is a guy there that is making aftermarkets that look exactly like the factory mags. His retail cost is $75. I'm pretty sure that was it.
You may have to search for : PatriotArms. I believe that was his name. He posts about once a week selling his mags. Other guys there have bought his mags and say they preform flawlessly.
The mags are parkerized rather than powder coated.