Having obtained a set of four new brake cams and two new front brake levers, I set about fitting them to our four spare back plates. The cams proved to be slightly non-standard. Unfortunately the cotters went right through without locking the cam shaft in the brake lever. The cut out was shorter than the original cams as well, by about 70 thousands of an inch, the difference is visible in the photographs below. This led to difficulty in fitting the felt bush. in the end I cut down the felt and used a vice to compress it into place.
Some modified cotters were obtained that worked on two of the cams, but not the other two.
Eventually I came up with a fix. I cut slots into the side of the cam cut out with a cut down hacksaw blade. Then made a tapered fitting to drive into it.
An original cam.
With a bit of work with files, the job was done.