Caille Rowboat Motor:

The detachable rudder was an option. This is the only original one I have or have seen. There have been a small number of reproductions of the original made.

Rudders were seen as necessary by some early outboard owners because a rudder provided steerage when the motor was turned off. They wanted to approach the dock safely (and not make a fool out of themselves).

With Caille's 5 speed motor you didn't need to shut the motor off while docking, you could shift into neutral or reverse....and that is why they didn't sell many detachable rudder options as most folks bought the 5 speed which became available in 1915, the same year the detachable rudder was introduced.

A "semi-detachable" rudder in a slightly different design was also available on Caille made Hiawathas (badge motor for Montgomery Ward).

1915
These links take you to the Caille site, which has all the Cailles, not just the rowboat motors.