Nice looking. That's called the "pyramid stamp". I've got 3 20" rides with that stamp. Well, actually the rides are 500 mm because they are metric. Same with yours. That's a 300 mm splash. My hats are 350 mm. You get used to it.
The smooth gradual bridge and killer bell (and most likely thick) are all part of the casting method used by UFiP. These cymbals are actually cast in the shape of a cymbal. The Italians are the only ones who do "cast" cymbals in that strict sense. Zildjian cymbals are more "rolled from a cast ingot" than "cast in the shape of a cymbal", but Zildjian have settled on the "cast cymbal" name for their superior lines so there it is.
As to the date, I'm afraid that I can't narrow it down to anything more than you already have: 1950s or 1960s. Not 70s as far as I can tell. By then you are into the Ritmo period which has different die stamped trademarks. In my spare time I'm trying to gather a bit of information about older UFiPs to answer that very question.
Rob Scott explains the origins of the pyramid stamp, but other than that his info is very sparse.
Enjoy.
