It looks like a Dunhill stem to me. At least nothing obvious jumps out suggesting otherwise. For what it's worth, I have a saddle-stem pot (not a Dunhill) with what I know is the original stem, yet the stem barely fit when I received it as an estate. Sometimes things can just get out of whack, especially if the pipe has gotten damp with smoking and was rarely cleaned (or ever in the case of mine).
If things just aren't lined up properly, you can gently heat the tenon and surrounding stem face and then insert it into the pipe, adjust it, and let it cool while in the proper position and that will help things. Just be gentle, both with the heating and with the "adjusting" or you could break the tenon or shank.