D's, the 1903 model M is a .32 cal, the 1908 model M is .380.
For the 1903's there were several variants, type I, Type II, Type III's, type IV. etc. A search at coltautos.com will give time lines I believe. You can also go to this link:
http://unblinkingeye.com/Guns/1903C/1903c.html
I heard that you could take a barrel from a .380 and put it in a .32 slide and use a .380 magazine IF the tolerance on the frame allowed it. This would mean that you could also take a slide AND barrel from a .380 and put them on a .32 frame and get a combo-gun.
This might be the case if your frame is numbered as a 1903 but the slide says .380 on the side. It could be a combo-type gun. See if there are any numbers in the slide in the void where the hammer would travel. Some slides had the last few digits of the slides serial number and later, 1920-ish, the entire serial number.
These guns are great and there is quite a bit of info on them if you search around. I just got into these in the last month or two and I'm still learning about the history and varients.
Good luck and please let us know what you find.