I use the Fowlers website a lot and they do list the saddles as having different part numbers for both the front and rear saddles for the k3 and then k4 onwards.
What the differences are, I could not tell you though, but if you know someone with a later bike, you could always try their seat for fit.
This is a link to the Fowlers parts page for the SV.
https://www.fowlersparts.co.uk/parts.../seat-model-k3