Documented on my site as 1972 onwards. The Made In Canada version:
http://black.net.nz/avedis/avedis-gallery.html#canada
and the No Country Of Origin version
http://black.net.nz/avedis/avedis-gallery.html#no-origin
Note that as I mention on my site, you can see that the No Country Of Origin version shares the "Trans stamp like" property of Turkish Cymbals being very close to the ZILDJIAN Co. So does the Made in Canada version, which may well be the same die without the country portion at the bottom (potentially a two part stamp as seen in the 1930s and 1940s). We don't know yet.
There are a number of hats from the 70s which are light, as well as some crashes and possibly some rides (although some of these might be 20" and 22" crashes lacking any ink -- the evidence isn't in yet). Zildjian made both New Beat style hats as well as "matched" hats (which simply means closer in weight).
There is lots more variation in weights in the 1970s than people give credence to when they operate from "stereotype" thinking rather than actually measuring the weights and seeing what is out there.
Of course, knowing there are some very nice light ones doesn't help you find them when sellers won't weigh them accurately. It's not really about the weights, but weights are a proxy for the sound when you don't get to audition them in person. :2Cents:
Enjoy them. And may I please add a copy of your stamp picture to my reference gallery? Thanks in advance.