You might try this easy way to measure chamber length using a 3" x 5" index card, rolled lengthwise, and slid into the chamber. It will partially unroll and expand to the chamber diameter. Gently advance the card until it stops at the end of the chamber (where the forcing cone constriction starts), use a pencil to mark the card at the breech end of the barrel, and repeat the steps to see if it always comes to same spot. Then use a ruler to measure the length marked on the card.
The chamber must be clean, and it works best with a new 3” X 5” card.
12g Smith guns from the beginning, unless marked otherwise, had 2 3/4" chambers; 12g Parkers 2 5/8". The Fox Model B should have "For 2 3/4" Shells" marked on the barrel.