Don, here is a chart of suffix letters. (Google "Luger suffix letters") Seems pretty accurate as it came from the Luger Forum.
http://www.lugerforum.com/lugermarkings/suffix.html
The more I consider the question, the more I lean toward the letter being added post-war when the subject pistol was refurbished. It's clearly obvious that it was worked on at some point but no way to know exactly when.
While there's no dot over it, I just can't see that letter being anything other than a "j" as no other letter, even if only partially stamped. resembles it. I've also seen suffix letters "i" without the dot. Just poorly struck of the die was broken.
I suppose we'll never know for certain. It's truly an oddity.