Hi,
I would agree to not do any alterations till the problem is identified. If the front barrel band is a correct 03 part on a 03 stock, it should work.
First, if you can carefully removed the front band, does the lug fit the rear of the bayonet and lock in the catch? Does the bayonet barrel loop fit over the barrel as it should? If those are fine, look to see if end of the stock is to spec, hasn't been damaged or altered.
Second, does the bayonet barrel loop stop prior to contacting the front of the sight base or does the bayonet barrel loop stop up against the front of the sight base and then the bayonet can't go the rest of the way on the front band lug to engage the bayonet catch? You indicated that the rear catch of the bayonet is almost engaged on the lug.
The bayonet sits on two points of the weapon, the end of the barrel (mounted with the receiver) and the front band lug which is mounted on the stock. If the parts are correct on the weapon and the bayonet is correct, then maybe the dimension distance of the front band lug, on the stock, to the front of barrel is off, too long. This was one area where weapon assembly tolerences were checked because the wood stock with fittings and the receiver with barrel must assemble together with a proper distance for the attachement of a bayonet.
Possibly loosen the receiver in the stock, by just loosening the handguard stock screws and see if the receiver will seat sightly rearward, pulling those two mounting points (barrel end closer to the lug) together, re-tighten and the bayonet may then seat. You say the distance it isn't much. Go slow and don't force anything together. It should just fit.
James