Snub
Yes, that's a diode. Looks like it
may be either a 1N5408 or a UF5408 ..
At least on my pic it looks like an '8' for the last digit.
1000V Vrrm 3A. Good enough .. if it is one.
If you look at the traces under the circuit board, it should be effectively wired across the output terminals, cathode [ end with the bar ] to the positive motor terminal.
In fact, as it has 2 signal diodes and a few other bits, you may well find the contraption is based on the generic 555 Timer Chip PWM schematic.
Similar to this one ... maybe not exactly, but ...
http://www.discovercircuits.com/DJ-Circuits/simplepwm2.htmIt is a common practice to parallel off two [ or more ] MOSFETs to get more grunt .. which
may be what you have..
BC