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posted it in AR mag forum. you guys might be able to give me more info.

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

No question that it's a Galil mag.  I question whether it's South African tho.  It appears to be standard Israeli production.  The South African mags I've seen have a different style floor plate than the original Israeli mags.

If it is Israeli, it will probably have a small 0 stamped near the bottom of the left side. It may or may not have some stampings near the bottom of the right side.  Military surplus will, the mags imported for commercial sales usually don't have the right side stamping.  The left side zero may be very faint.

As you mentioned, the Galil mag is very well built.  Mine are over 20 years old, been used 50-60 times each, still as good as new