There are a ton of variables to the 03's that can make them a curio or a serious collector. The serial number on the action will tell you the year of manufacture and if it is a good one or an early one that suffered from uneven heat treatment and are considered dangerous. If you have one over 800,000 in serial number, you have a good one. The date of the barrel should correspond with the date of the contract serial number. There were a lot of rebuilds and these are not worth as much. There are different models. I have several National Match 03 and some 03A4's which were the sniper rifles. The early models of the 03 had the sights in front of the receiver. The later ones the sights were on top of the receiver. There can be different grooves in the rifling in the barrel and different and unusual barrels. I have 2, 4, 5, and 6 groove 03's and all are correct. The 2 and 4 groove are fairly common. The 03A4's and the National Match are the high dollar collectibles along with the ever elusive carbine model. I only have one carbine and it's the only one I've ever seen but the NRA did an article on them many years back and they were only issued for the Philippine campaign for jungle fighting. The barrel length is 16". The fact that it is in cosmoline helps the value and kindly leave it that way. It may or may not be a rebuild as they packed them both in cosmoline but it adds significantly to the value. If you can post pics and serial numbers, inspectors marks on the wood and barrel, I can pretty much tell you all you'll need to know about it and maybe even the value.
And let's not screw Uncle on this thing. You need to be straight up with him about the value. He may just hand it to you anyway but he deserves an honest answer.