In his book Knibbs states that all the Military Patterns were in either .177 or .22, the quantity of either depending on the batch.
I suppose it is conceivable that the odd .25 barrel was grafted onto a M/P from a .25 Imp. Mod.D of 1908, but I think it unlikely.

What is definitely nonsense (IMO) is the fact that there were 100 made in 25 calibre. There were only about 450 M/P made in total, so 100 represents getting on for 25% of the total production. The M/P is a rifle which has survived well, I personally have recorded 42 serial numbers, not one of them in .25.

If there had been 100 made then others in the larger calibre would have come to light by now, simply by the law of averages.

So forget it, this particular one may be right in that it may have been one of a very small handful put together as an experiment for example,but even this is unlikely otherwise it would have appeared in the records and another would have been found by this time.
Rather like the Short M/P, the existence of genuine examples of which are very much in doubt, and that rifle was actually advertised and priced!

A single unit in .25 suggests a non-factory one-off, the provenance of which therefore will always be in doubt? I would however be delighted to be contradicted if evidence exists to show otherwise?