Possibly related, on my Yamaha 4 stroke OB engine, its an F150, the "F" being for FOUR stroke, 150 HP. That's not a guess, thats what the dealers use for looking up parts, etc...
As for the Z, ??
Back in the old days early 70's AT1's (enduro's) mid to at least late 70's MX racing dirt bikes, "YZ's"