That style Evinrude pump model P was made 1915-1929. Yours pictured is a later one, maybe ’26-’29.
The ’15-’17 had 3 legs from pump-to-powerhead and it used a normal brass muffler can. The 17-’22 had 2 legs angled towards each other at the top. The ’23-’29 had 2 vertical legs. The ’17-’24 used a cast iron vertical exhaust manifold/muffler. The ’24-’29 used a cast iron horizontal exhaust manifold/muffler. In 1928 the 2HP version could be had with a brass pump.
The P style pumps came in 2HP or 3 1/2HP versions.