Possible replacement for faulty Heljan motors, for example. Or cheap alternative to Mashima for kits. Some Mitsumis come with a worm (pitch unknown), so that will need to be extracted if you're using your own preferred geartrain.
Dunno if this will show up, but here goes:
This is a blown-up crop of the original photo, the copyright of which belongs to SLS.
There's also six rivets on the door itself, in a hexagon formation.
It might actually be 35 rivets because there are a couple that seem to be hidden by the smokebox door hinges.
I've taken a close-up photo of the original photo, and will see if I can blow it up even more to verify the count.