The Waterman company was bought by the Arrow company in 1917. Arrow continued to offer "Waterman" rowboat motors. See Arrow for motors produced after the buy out.

http://www.angelfire.com/mi4/hartsmith/thesnows.html
Two full page ads for the Porto from 1914. CLICK IMAGE

Note the vocabulary used by the 1907 Waterman ad above. They have no name for the new type of motor that is detachable, still calling it a marine engine (which designates an inboard). A very similar, but not identical, 1908 ad is below the PORTO ad.
Click on the examples below to view photo albums.
example 1, 1906
example 2, 1907
example 3, 1908

example 4,
1912 C-13

#433
example 5, 1912, rear cylinder

example 6,
#12008

example 8, Waterman Sweet
(a badge motor)

example 9,
Waterman C 16
example 7,
vertical,

example 10,
Waterman C16
#16173

This is a 1907 Waterman crank handle and period buzz box.
The clamp was for your boat...once attached to the boat's skeg (which most had back then) it was the receiver for a protuberance on the motor.

To see more closely in both photos and catalog images what I am describing, go to this page.

Below: 1914