Oops. Looks like Gewehrfreund beat me to the post button.
Your rifle sure looks like a 12/15 to me. I have two of them and yours is identical to my eye.
12/15's should have a serial number starting with the letter 'P'. I don't see the 'P' in your photo so that is one question.
The barrel markings are not familiar to me except for the markings on the top of the barrel as shown in picture 3.
The 8/10/51 date and the 'Jessops' name, on the bottom of the barrel, might indicate a re-barrel in '51.
Nice looking rifle.
Good luck with your investigation and shoot straight.