Great stuff from the magazines, Don
I have a couple of the nylon versions of the MRRC braking gears. The contrate has a large boss on the outer side and ramps or angled notches on the inner.
The notched collar is secured on the axle (by grubscrew) and the contrate is free to rotate on the axle. On the over-run the collar (and axle) is pushed slightly sideways against the side of the bracket.
As Eddie said, they were made for the bracket on the open-frame motor, the 3 or 5-pole. They could be used in other brackets but then were probably more difficult to set up.
The pinions were brass, whatever the ambiguity on the pack. There are two contrates in the photo, of course, 24 and 32T.
The metal one looks different, from the part of it on the 1/24 car. Perhaps when used with braking a sleeve or grub-screwed collar would be needed on the outer side of the contrate.
In Vic Smeed's 1965 British book Simple Electric Car Racing
he describes these systems (ready-made and fabricated) and says the method "has proved extremely successful. Used by the 1964 Sports Car Champion on both axles, the car being four-wheel drive."
Maybe Tony or another historian can tell us of that.