When the real experts get here they will be able to give you a ton of very good information.  Until then, I'll limit myself to some bullet points that I'm sure every one will want to address.

First, did you buy it?  Do you have a picture of the extra barrel?  The serial number would indicate that this piece is from the last few months of production which would put it far later than the Constabulary or School rifles.  Those that were actually the constabularies were almost all made from 1899 carbines which were all produced with 22" barrels.  There would be no need to alter barrel length to produce new, special design.  What would have been needed was a new stock which would be different from this one.  The Constabularies had the barrel trimmed at the muzzle - not length, but diameter - to accept a bayonet.  The hand guard on this rifle looks like it was made for a Model 1894 which was shorter than the standard rifle.  The end nearest the muzzle looks to have been "shaped" a little to give it a finished appearance so, I guess it could also be an 1896 rifle hand guard that has had some work on the end.  The experts can confirm by dimensions and maybe by rivet spacing l think, so we'll have to wait for their input.  There will be a lot of information coming which I'm sure will be helpful for you.

Now, the best part:  The rear sight you've pictured is (or at least the leaf is) graduated for the 1896 Carbine sight.  It carries the correct appearing "C" and the base should have a similar "C" as well. You can check that detail.  Until we consider the rear sight, the rifle you have (?) was probably a 'parts' gun.  In fact, it still is a parts gun.  But, the rear sight is very special part and on a good day worth the whole asking price.  The hand guard - if correct, is probably worth a nice little amount as well.

Now we can wait for the opinions to come along.  But it seems like a fun piece anyway and might even be fun to shoot.