I second Zippy's point....you need to have a high end gun dealer look at it / or at least get some good pictures so they can evaluate it.
Barrel length - might be an issue / barrel lengths today vs 1957 have changed ....or trends have been going toward longer guns with 30" O/U's being the optimum today.../ if yours is 26" ...its a little short ...if its 28" it may not matter.
12ga barrel is one weight....
adding the tubes for 20ga ...might add 10 oz - 16oz ...which change the handling of the gun ( swing characteristics, etc ) .../ it'll be different with the tubes in it ...but a guy might want to just shoot the 20ga in 20ga and 12ga events...if he or she likes the way the gun handles with the tubes in it better than as a stand alone 12ga.
grade 1 isn't too fancy / but they still put some nice wood on them for the time...
There might be a lot of guys that would want this gun ...../ pictures would help.