Don't worry. Zenstat is a Kiwi. They talk funny down there, probably a result of being upside down and having all their blood rushing to their brains. :)
Zenstat was born and raised in California, but went native in New Zealand some time later. He has not always lived uʍop ǝpᴉsdn.
Focusing on the stamp itself here are differences which have come to my attention. Note I didn't spot these. Other people told me to have a look for them. All I've started to do is try and annotate photos so we can have a better chance of knowing that we are talking about the same thing.
Beginning at the 60s stamp just so we have a frame of reference:
[img]http://black.net.nz/cym2014/60s.jpg[/img]
You can see the red arrow and circle around "three dots". People seem to think that the 50s stamp and the 70s stamp don't have the "three dots" and that means they can be confused easily.
In the later 50s we have the stamp without those dots:
[img]http://black.net.nz/cym2014/50s.jpg[/img]
But look carefully at two features. Number 1 is that nice vertical alignment I point to in red. Number 2 is that the font weight (or stroke weight) is heavier for two of the elements, the ZILDJIAN Co and the Ottoman part of the text. The AVEDIS is lighter. The rest of the text is in the same weight as the Avedis.
Compare the 50s to the 60s and you see the same vertical alignment and stroke weight differences are there. Hint: if you can't pick up the stroke weight differences relax your eyes and just go for the overall impression rather than focussing hard.
Now a 70s stamp
[img]http://black.net.nz/cym2014/70s.jpg[/img]
I've annotated the same two places. You should be able to see that the vertical alignment is no longer there (red arrow) because the MADE IN USA is more spread out. The second feature (2 green arrows) is that the font stroke weight is the same for all the elements. I find this easiest to see when comparing "ZILDJIAN" with "AVEDIS".
These things are relatively easy to pick up in "eBay/Crasigslist" poor quality pictures.
Failing that you fall back to the tried and true "the 70s stamp is often very light or poorly struck". Alas, I don't know anybody who has tested this theory and told us sample sizes and given us confidence limits for using it. So we don't know how well this distinction works. Is it correct: 8 out of 10? 99 out of 100? I'd want to know the accuracy of the method if it was the only criterion on which I had to base a purchase.
Now the complications: I haven't mentioned that there are 4 other 50s stamps (at least), a later 60s stamp, etc. Most fun of all is that one of the 50s Large Stamps has the dots. Oh joy. Trouble for identification. If you go back to Drumaholic's article I linked to you will see these and also see that they are different heights. Alas, sometimes getting a seller to measure stamps accurately is as hard as getting them to weight a cymbal accurately. But these things might put some more tools in your toolbag which will help you tell the difference.
I've also heard from two people who have counterexamples to the 50s vs 70s alignment attribute. In other words one claims to have a well identified 50s cymbal which lacks the vertical alignment. The other claims to have a well identified 70s cymbal which has the vertical alignment. So what next?
In order to assess how well the alignment or font stroke attribute does in predicting 50s vs 70s all on its own a statistician (such as my good self) wants to know sample sizes and how many there are which fit in the 4 different categories. I could (on no evidence) argue away these two as misidentifications of the decade. I could instead treat it as a statistical issue which says "no problem with some counterexamples if they represent 1 in a 100 or fewer" because that would still leave you with a great rule of thumb. But nobody to my knowledge has ever told us about how many of these they have looked at, and how many did or did not fit the expected pattern. Without that basic reporting of information we have no idea how well (or poorly) any of these methods might work.
If this does it for you I'll hold of the general pics of lathing differences between 50s and 70s (partly because I have even less faith my understanding of the differences and the quality of my illustrations).