Firefighter,
The trick on a gun like this ...is to find a guy that wants it as part of his collection - or better yet, wants it to shoot.
If it is 28" barrels ...that's a positive thing / will attract a bigger group of buyers....but a lot of these older guns were odd lengths, like 26.5" ...
But guys like me ( 6'5" and 290 lbs ) went to O/U's like the Browning XS Skeet model Citori with 30" barrels 10 yrs ago...and a gun, even a superposed, in 26.5" or 28" just doesn't have any value to most of us ..unless I bought it for one of the grandkids to shoot - at the right price, a guy like me is only going to be a buyer at $5K or so. The other thing going against it ..is it is probably fixed choke tubes...probably Skeet....so it isn't a gun that could easily be used for live bird hunting, or sporting clays...so again that might drop the value.
I don't think the tubes came as part of the set from Browning new in box...but the original owner might have special ordered the gun - then sent it to the tube maker/tube fitter...and had them fit. Kind of depends on when it was done...and I was only 6 yrs old in 1957 - so I don't know who was making them in 1957 for sure.../ or the tubes may have been made much later by someone like Briley or Kolar and fit to this gun later.../ you'd have to carefully inspect the tubes to really know.
But all of this stuff means you need to really have someone look at it - that understands what a Skeet set is ....and help you go over the pros and cons of how this gun is configured / figure out what its worth ....maybe the value is $15K ...or maybe its only $5K ....
New in the case, and unfired, can be a big deal....but 99% of us are going to shoot it ..../ so in my opinion, it helps you sell it ...but it doesn't raise the value a lot.
A lot of Skeet shooters spend a lot of money ...on high end guns...like Krieghoff and Kolar....or Blaser, Perazzi ....and the "pedestrian guns" like Browning Citori XS Skeet models (which I shoot - in all 4 gagues ) - or most of the Beretta line of guns - although Beretta has some high end guns too.
I spent about $ 13 K on all 5 of my Citori XS skeet model ( bought two in 12ga so I had a backup gun ) and one each in 12ga, 20, 28 and .410 .../ negatives - my guns are a little different ...20ga, 28ga, .410 are all built on the 20ga receiver ...so they're all at 7.5 lbs and swing the same ...12ga is almost 1 lb heavier ...so I weight the other 3 guns to come up to my 12ga weight.
If I had a single 12ga gun ( like your superposed )...maybe its an 8 lb gun now...adding a set of tubes might make it 9 lbs....so I'd want weight added to it (under forend / and maybe inside stock ) ...to get all 4 gagues up to the 9lbs....or I'd just shoot it with the 20ga tubes in it ...in 12ga and 20g events...with 7/8 oz of shot ..and live with it.
The best system of all....get a 12ga barrel - have it bored out ( to fit the tubes) ...call it a "carrier barrel" ...now the carrier barrel with tubes in it weighs 8 lbs....and you have a stand alone 12ga barrel and its 8 lbs...one receiver ...triggers are all the same, its a perfect system for competition. Far better than having 4 stand along guns like I have ...or a 12ga that is tubed like you have....
here are my standard Citori XS Skeet models....
http://thefiringline.com/forums/attachment.php?attachmentid=82011&d=1339792349
Here are a pair of my Citori XT ( trap guns ) ...
http://thefiringline.com/forums/attachment.php?attachmentid=82013&d=1339792349
one is a standard grade IV ...one is a custom Turkish Walnut grade VII maybe...?? that I bought used....
But that grade VII I have, is even pretty plain next to some of the Krieghoffs and Kolars
But a Krieghoff ....with a carrier barrel and tubes ..and a stand alone 12ga barrel...starts at about $ 15K ...many go up to $ 25K with nice wood ( real nice wood and engraving ) which will make any grade I Browning, even a superposed look pretty plane jane...
Even my Five XS Skeet models, all nice guns, grade IV wood on them ...all Citori models made in Japan by Miroku ....../ but your superposed has some nostalgia to it ...little different feel to the grip area, maybe a little lighter...so its different....but value...who knows.....