Spanish, IMO, most likely off a 1960s S2. It's a Spanish-style catch at the back and the front plate that bolts on the seat is also a give-away. Also, the Spanish bench seats of the period had the in-out curves similar to a TV S1 Aquila....
funny you should say spanish as i think i may have bought it for a spanish bike. looking at the way the cover goes down at the front though it looks more like a series 2 style?
i have been looking at pictures to see if i can see this type of seat and only the sries twos seem to have that front flared part