it must be a Service Twin ( I have exactly the same as yours and mine has model 4161 on the ID plate )
based on two things;
the cavitation plate is a little bigger then the Fisherman
and the little extra outlet on the alu. muffler
makes it ( IMO ) a Service Twin 1936 with 4,3 hp
btw. I have a Fisherman too ( name on the tank decal ) but on the ID plate is model 476 and that
belongs to the Sportwin 1933 with 4 hp
so you can make a choice with all the info here on the board
😉