N15 Engine number stamp

I am looking at a N15 project and the engine number has "P11" stamped at the start of the numbers. Does this mean that the bike has been fitted with a different engine? I'm afraid I do not know the frame number.

On the down tubes/headstock for the N15, there is a lug with a horizontal hole in it and the brackets for the passenger pegs have threaded holes in them about an inch in diameter .
I don't believe the P11 had either of these.

Thanks Frank. The frame and cycle parts are definitely N15 but my question was about the engine stamps saying P11. I was wondering if N15CSs were fitted with P11 motors.

Nope, the engines were stamped either N15 or G15. The G15 being the Matchless variant of them. As far as I know, the frames were stamped on the headstock with just the last 6 numbers of the engine number. Only the the engine had the N15cs stamped into it. I have looked at, and wondered about frames being stamped on the seat attachment lug, but never found a definite answer. Walridge Motors https://www.walridge.com/ has a lot of information on the highbreds. Give them a call