Below is a photo of the caliber marking. The double line engraving indicates to me also of a later marking. There is also a link to the six or seven photos he sent of the proofs. From the styling of the gun it looks pretty early to me also. According to the owner, JJ opened the chambers to 2-3/4" and blessed the use of any modern ammo of that size. JJ is the one who confirmed the chambering and put the caliber marking over the chamber.
http://s396.photobucket.com/user/Vol423/slideshow/12ga%20and%2093%20by%2074%20drilling
