IIRC, XD and XM are wrong and should be CDXC and CMXC instead.

I was wondering how long it would take...

It's debatable and I don't much feel like debating it. It's hard to say what's "right" and "wrong" here because it's not like there was ever an accepted standard of use; just convention. I think there are examples of both in ancient usage but you'll probably be able to make a stronger case denying XD than XM.

And, while I'm revisiting the subject, I'm surprised no one has so far mentioned that I've forgotten XM...

-sauoq
"My two cents aren't worth a dime.";