Thanks, Kiwi, there's a lot there and your explanation is good. I can now understand how the idea works.
I think BA60 is a general Bridge Rectifier number - you will need to get one which can take the voltage and amperage. 12v is probably no problem - most will take that, but I don't know what the max amps are likely to be....
I do not understand the difference between the Supermarine and the Supermarine Double Special. In my collection so far I have a Meteor and a Standard, but this is the first big motor I have. I note that some pictures show the big twin-coil motor with brushes working from the side, and some working axially - also some motors have a one-pole reverse connector. Mine has axial brushes and no one-pole reverse. It has two connector plates to join the field windings to the brush windings.
I have sketched up a couple of simple drawings using Paint to illustrate what I think is happening. The first shows a rectifier in normal use - an AC supply is connected across two terminals, and DC (with a lot of ripple!) comes out of the other two. As the AC supply switches polarity, the polarity of the DC output remains the same.
The second sketch shows a field-wound motor connected to a brushed speed controller with reverse facility. The speed controller can change polarity, just like the AC supply (though not so often!). When it does, the polarity fed to the field coils will change. But the feed to the brushes, coming off the rectifier, will not change. This is the correct situation for a field-wound motor to reverse rotation....