Specifically, the black ones.  I know they are marked with different numbers, but is one better than the other?

They are pretty much all the same. If they are rarer they go for a dollar or two more, that's all. Some are different colors, so look at the pictures close.  

Quoted: collectibility is the difference. they are all 100% reliable.

The reliability of the Kvar mags is not guaranteed. I have approximately 30 various k-var AK-74 mags, they work in some AK's, and they don't in others.

That is not to say they cant be modified or altered to work correctly, it is a buyer beware issue. I have also heard that some of the K-var AK mags are seconds.

Quoted: Quoted: collectibility is the difference. they are all 100% reliable. The reliability of the Kvar mags is not guaranteed. I have approximately 30 various k-var AK-74 mags, they work in some AK's, and they don't in others. That is not to say they cant be modified or altered to work correctly, it is a buyer beware issue. I have also heard that some of the K-var AK mags are seconds.

Is there any way of telling if you have seconds?

I bought so many AK mags from K-Var they send me a letter asking me for my dealer info

If any of them are seconds I can't tell. Still, I didn't  buy them all so let the recipients of the seconds speak up and post pictures if it is a big deal.  It takes alot to trash an AK mag, cosmetics aside.

The only issue with them is the Bulgarian mags are slightly oversize, probably on purpose. So they might be hard to insert into your rifle the first 10 times, as the magazine well scraps away excess soft plastic.  Once you do this they are fine and are mated to your rifle.

East German orange mags are hard plastic and fit a bit looser, so you don't get any shavings.

Both types are good mags.