I was under the impression that an AK is an AK pretty much. Recently saw a "quality" Russian made that wouldn't take a Polish or Turkish mag. Any idea why?

Didn't they make the mag opening a different size during the "assault weapons ban" days? I seem to remember something about this but not enough to say for sure.

Some receivers are for single stack. Not hard to retrofit with a Dremel...

As for mags... there are only "normal" or "single stack" to the best of my knowledge.

Do an Arab reload...hold the bolt back and try insertion,.
Doesn't fit or doesn't lock?
One is easily fixed.

"Recently saw a "quality" Russian made..."

Saiga perhaps? Have to convert those to take the excellent ComBloc standard magazines.

Might have been, I think it had ishmash or something and made in Russia on it.

What does the conversion entail?

My thoughts exactly. The Saigas have to be converted to work with double stack Hi cap mags.

There is some variance in AK mag widths. They probably redid the mag opening using a slightly narrower mag and wanted a very tight mag well to magazine fit.

50s and 60s russian ak are nice, beyond that there's better for the buck.