They are the ones, Ginge, but much smaller than a matchbox and cost about £5 in our, sadly, funny currency. As has been said several times, these are rectifiers to rectify the current from alternator AC to battery DC and are just a simple replacement for the original plate-type rectifiers that were fitted by the factory.
You just need to fit these replacement rectifiers if you are still running a six volt bike with its original up and down wiring to the complicated light and ignition switch. If you are converting to twelve volts, or doing away with the old switch wiring and keeping to six volts, then you will also need a regulator. This can be done on by fitting an expensive electronic regulator/rectifier instrument, that also does the rectifying, or, on 12 volts, by just also fitting a Zenor diode like BSA did when they went to 12 volts in the '60s. This works by monitoring the battery voltage and, in simple terms, 'short circuiting' the excess to earth as the battery voltage creeps up over 12 volts. It sounds harsh but it works very well in practice, was good enough for all the Brit manufacturers back in the day and still works for me.
Ray
