the magazine pictured below has no identifying marks whatsoever. however, we loaded it with 35 rounds of 5.56 ammo, took them out, and re-loaded the mag with 30 rounds of 7.62x39 ! the gun store near my house (Bob's Gun Shop in Georgia, VT ) aka "my hang-out" , has about 12 of these magazines. what are they? where do they come from? are they good magazines? they seem to be made of thick gauge steel as they are a bit heavy and very well put together. any help will be appreciated.

Looks like a Galil mag.

Galil, or maybe Valmet.

That's a Galil 5.56mm magazine, maybe made in South Africa for the R4/R5.  Maximum capacity is 35 rds.

It's NOT a Valmet magazine, and Galil magazines, while able to lock up into a Valmet, shoult NOT be used, as they will misfeed and may cause the round to nose dive while feeding, driving the bullet back into the case and increasing pressure into the danger zone.  Valmet 5.56mm magazines hold 30 rounds BTW, not 35.