Are you outside the USA by any chance? The ink on the underside of the bell is found on export cymbals. That doesn't mean that the cymbal hasn't been shipped back into the USA some time in the past and then ended up in your hands. The ink on the underside of the bell includes the diameter in cm and inches, and usually has the Avedis Zildjian logo (in Ottoman) between these. From my site:
[img]http://black.net.nz/avedis/images/nb-other-ink.jpg[/img]
The 17" diameter is uncommon compared to even diameters (like 16" and 18"). I'm not sure where I'd draw the line at calling them "rare". They have been around since the 1930s. I've got 76 17" cymbals recorded in my tracking database, compared to 406 18" cymbals and 140 16" cymbals. I've got enough data to provide some useful context to 17" weights which are found here
http://black.net.nz/avedis/avedis-prices.html#17
If you are equating "rare" with "more valuable" that isn't necessarily the case. The relationship between prices and diameter (and the interaction with things like production era and weight) is more subtle and nuanced. I haven't made a one on one comparison of, say, 16" 17" and 18" prices. When I had look at the question of how prices relate to diameter (including odd diameters) for larger cymbals the answer was...interesting things happen and relatively rare doesn't automatically translate into higher demand thus higher prices. This is hardly a surprise if you believe traditional microeconomic theory. There are lots of intervening variables at play.
http://black.net.nz/avedis/avedis-prices.html#big
I second Mike's observation that your cymbal wears what we call the 1960s trademark stamp. It also has 60s looking lathing style and hammering style.
Did your exhaustive Google search take you to my site? I only ask because I wondered if you were exhausted by reading my site (another simplification/overview is called for) or by following all the Google links. ;)