Looks like your rifle was originally produced in early July of 1898 when production was ramped up for the Spanish American War. Check your barrel for any markings. It is probably a shortened original barrel from 30" as you suspect. But it could also be a replacement. Some 1903 Springfield barrels (24" long) were used in some rifles' later life as a replacement. And there are other possibilities.

You will get more responses than this. Just though I'd kick it off.