They leaflet that came with them told me they were the same servo as the 8411, but were suitable for use on 6V rather than 4.8v.
This is another JR 'oddity'!
The 8411 is also able to run on 6v but the UK importer apparantly knows better than JR. Most of the official UK JR servos I have bought have a leaflet stuck to the outside of the box (i.e. not put there by JR) restricting the voltage. I guess the reason the 8425 is not restricted is simply that it is not a servo produced for the UK market (and I'm guessing possibly not supplied by Macgregor).