What is the history of a T Baker shot gun 12Ga side by side?

You can check the yellow pages for gunsmiths or browse the tables at gun shows. You might find something that will fit or can be made to fit, but you will never find anyone with a supply replacement parts. If it was made prior to 1900, it probably has twist steel barrels and was designed for 2 1/2" or 2 9/16" black powder shells with rolled ends. Using 2 3/4" smokeless ammunition with crimped ends is NOT recommended. Clean it up and hang it over the mantle as it is. Your granddad's homemade stock is part of its history.