If there is genuinely M1900 written on top then it would have been a 6.5x54 originally as the 8x56 never came out until 1908.

However the width of the body of the M1900 and M1903 is smaller than that of the M1905, M1908 and M1910 as the 6.5mm cartridge has a smaller body diameter than the 8mm cartridge. Therefore it would be a lot of work to convert the M1900 to 8mm if it is even possible, especially when you can buy an M1908 fairly cheaply as it was the least popular model and that is still the case today.

Matt.