The TR3 & 3A have a quite different caliper from some of the TR3B-Early TR6s.
The early caliper has two versions: one with internal fluid drillings, and
one with an external fluid pipe; they are interchangeable.
Note that the caliper adapters that bolt to the vertical links are different,
early and late: i.e. 3 & 3A from 3B-6. The later setup also runs
with a slightly smaller disk.
Rev. limiters: The one on my Electromotive is too hard on the motor
to use (We tested it on the dyno) The "soft touch" limiter on my
old MSD was MUCH better. It could be that Electromotive has improved
their product. I have not tried the MJB unit.