CDNN Investments out of Abilene, Tx (as advertised in the Shotgun News and Gun List) are the only "discount" source of SIG P-210 magazines I am aware of. Yes they are factory mags but, they are discounted, some.....
Unfortunately, there is such a small demand for those mags, they are apt to be expensive forever since volume will never pick up enough to force prices down or someone else (Mec-Gar, USA, ...) to put out a cost competitive alternative.
Sid
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Tx FFL