JDW, I gave John Houchins a bunch of info on serial numbers as well as gun pictures of the Baker and Baker Smith era. Somehow, some did not translate correctly into his book. It was a massive project and I am amazed he could compile what he did. If you follow my guide above, you will have the "most" possibly correct information.
In the early Baker guns and Baker and Co. guns, there were several types of forend fasteners. Finally, the J spring won out and is found on guns by several other makers as well.