His is indeed single keyway, but it is of different Pitch than most that we've seen for replacement. I suspect that he may have one of the oddballs, or possibly someone before him swapped gear boxes.
You might also check with Monarch. They might have a used gear for you. Based on square-dial Parts Sheet 101, it looks like the gears and most of the guts date back to the time of the early MGs, so may be common to all back-gear units. I think that the gear you need is EE-2208.
As Bill suggests, you could buy the other gearbox, and swap parts to get one working with the correct input and output setup. The output shaft also varies - some take a smooth bore pulley, others splined, and there are a couple varieties of splines.
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