Re: fox doublebarrel .410
Your 410 side by side Fox is a Fox in name only. When Savage Arms purchased the A. H. Fox Gun Company in Philadelphia in 1929, it acquired all current inventory, the Fox name, etc.
And you're right, US President highly praised his FE grade 12 gauge Fox that was made around 1908-1909. By the way, TR's Fox just sold 2 weeks ago at Julia's Auction House in Maine. Winning bid was $750,000.00. Add 10% buyers premium to that price.
Without having your gun in hand and no pix but your description "in fair condition, still really tight, blueing is ok", my guess as to its value would be in the $300 to $500 range. Assuming it works OK, just has cosmetic issues.
The Fox model B made by Savage in the 1950's and 60's is just a dressed up Stevens 311.
Anyone else care to help Fairtradder out with the value of his Fox 410.