That's a wild potbox! I'd guess it's actually a big rheostat, or an array of discrete resistors, rather than a carbon-type infinitely variable sliding resistor.
Another thing I noticed is the motor itself looks fairly small. Diameter is similar to ours, but length is much less. Keep in mind with our motor mounted with the tranny on the 3-cyl mounts (as this one is), our motor almost touched the right side frame.
For comparison - this is our motor in the 4-cyl offset position (offset to the L is about 4 inches). Motor is 15 in. long:
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