"I don't see a 'trapdoor' in the buttstock. Is that common on the M1900? "

A beautiful rifle to be sure.

I suspect that the stock is not a factory original but a replacement, possibly by the firm of John Dickson & Son. I'm not aware of any factory stocks with an ebony or other wood fore end but that doesn't mean it wasn't so. Not being an original stock could very well explain why no butt trap with pockets for a sectional cleaning rod and two cartridges. I also don't think the express sight are factory original.

Having speculated as I have I may have just made an ass of myself as my knowledge of MS rifles is small. I have four factory original rifles from before WWI, a 1903, 1905, 1908 and a 1910. None of these rifle sport express sights. One has a half stock but it is not like the rifle above.

It could be very much Like Mauser though, for the right price they would build what you wanted.

Vlad