Hi Gas Mantle,
I wonder if the reversing gear is an external hand operated piston valve, which swaps around the supply and exhaust. This will not work on the small engine which appears to have slide valves, but the larger engine may have piston valves which lend themselves to reversing this way.
Unfortunately the film did not say much about the engines or how they work, and in fact seemed surprised that the larger engine did reverse. The few quick views of the top head did not make it easy to determine what kind of valves were utilised.
MJM460