You're not talking nonsense at all, 'Nando,
I can't solve the riddle either, though, apart from the fact that 25 mm is definitely not a standard musket caliber. Regular matchlock and wheellock muskets of the early 17th c. normally had smoothbores of ca. 19-20 mm.
We just know way too little about these special pieces.
Best,
Michl
P.S. The fuse is coming your way next week.