In order to value a special order gun, it would be best for you to get the gun to someone who knows them very well so that they can evaluate it. Pictures would certainly help people here to give a more detailed opinion. 20" 33 WCF are certainly not common and could be modified after it left the factory. Getting the factory letter info from the museum would be the place to start. If they confirm the special features, that's a big step.
Is it a 20" short rifle or a 20" carbine? Does it have any other special features (pistol grip, checkering, engraving, special sights, upgraded wood, etc)? And, probably most important, what condition are the metal and wood in? Has it been modified or refinished?
If you want to email pictures, pm me your email address and I'll send you mine.
