The barrel should be 28" from breech face to muzzle. If you measured from the front of the receiver, 27" would be about right. It most likely started life as a MkII 3* or Mk II 4*. I would not put any money into restoring it with that cut receiver. However, aside from bands, good full length barrels are the most difficult part to find. There are quite a few Mk IIs with shortened barrels around. I would purchase one of those and transfer your barrel and, perhaps, a few other parts over to it. Mk II barrels, with the exception of the 2*, are pretty easy to remove.
And then take the remaining action, rebarrel it to a better caliber, and make the aforementioned fine custom rifle. Perfect solution! If I weren't on the other side of the border, I'd make you an offer on the action right now.