My attitude to a story like this is: if it isn't in "The Crown Jewels" then it is, putting it very politely, hard to believe. And "The Crown Jewels", by Dana Jones, is quite specific on the M94 carbine and Mauser deliveries:
7 Aug. 1894. Date of adoption as 1894 °ars karbin.
13 Sep. 1894. Mauser applies for building permission for "another factory building on the site of the monastery garden" to handle the expanded production, later termed the Schwedenbau (Sweden Building).
3 Mar. 1895 Two Swedishinspectors arrive in Oberndorf.
6 Apr. 1895. Designated name changed to 6.5mm karbin m/1894.
Ammunition designation 6.5mm patron m/1894.
So the designation specifically included 6.5mm for carbine and ammo. Before anything was delivered at all.
24 July 1895 A further 9 officers and gunmakers arrive from Sweden.
7 Sep 1895. Swedes are checking the gauges and patterns.
27 Sep 1895. Test firing of first 20 production carbines.
19 Feb. 1896. Last carbines finished at Oberndorf.
The timetable makes it clear that the Swedes were on site checking everything before anything was manufactured, let alone delivered. I see absolutely no necessity or window of opportunity for Mauser to have needed or wished to deliver anything other than the specified carbine. In fact, in recognition of the fine performance in completing the contract within 6 months, Paul Mauser was awarded the "Commander Cross Second Class of the Wasa Order" by King Oscar II on 7 Aug 1896!
However, I have seen other Swedes offered with 7mm barrels. Genuine m/1894s in good condition are worth several times more than a Spanish gun of the same vintage, so I suspect that they are mixmasters of Swedish stocks with Spanish barrelled systems to create something more desirable. Hon y soit qui mal y pense as the cynical guys used to say in the Middle Ages.
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Did he have his fingers crossed while swearing? And did he swear on the bible or Hoyle´s Book of Games?