Just noticed that your 20" also has the word MEDIUM in the font with the sloping out M. That's another sign of early 60s for that particular that MEDIUM ink stamp, although that ink did last until at least the later 60s. Here it is on a Made In Canada
[img]http://black.net.nz/avedis/images/20-medium-dance.png[/img]
This is the Made in Canada trademark which doesn't have the Country of Origin line included. That puts this ink as continuing on until 1968 to early 1970s.
Model ink stamps changed font style later on (not sure what year), although it wasn't until around 1982 that Zildjian added the diameter info underneath. I've started collecting weights for all production eras of cymbals with model ink as part of an effort to document the differences between the different models. If you are able to get a weight for yours (grams or lbs/oz id doesn't matter) I'd love to add that to my data.
Here is a recent compilation for 20" A Zildjian cymbals with model ink and 70s stamp (except where noted on the trademark stamp) in increasing weight order (mostly)
THIN 1620g
THIN CRASH 1695g (60s stamp on this one)
THIN RIDE 1748g; 1755g
THIN 1814g (60s stamp on this one)
THIN CRASH 1944g
MEDIUM THIN 2010g
CRASH RIDE 2080g; 2114g
LIGHT RIDE 2130g
MEDIUM RIDE 2260; 2290g; 2360g; 2380g; 2400g
ROCK RIDE 2550g; 2646g
MEDIUM RIDE 2676g; 2716g (80s) (these are 200g heavier? weighing or data entry error?)
DEEP RIDE 2926g
PING RIDE 2740g (60s stamp); 2850g (80s ink); 3000g; 3030g; 3100g
EARTH RIDE 3675g
Similar compilations are possible for other production eras, but I happened to do this one first in order to answer a specific question about a cymbal with a 70s stamp. Model and weight ink on older (pre 70s) cymbals is pretty rare (due to overzealous cleaning) so I've looked at 83 to get the few with model ink listed above.
I'd love to add your cymbal weight to my data for the next iteration.