This was discussed here earlier this year (also in subsequent posts and there's a link to an earlier discussion of it), doesn't look like everyone agreed or a conclusion reached though. Don't know if it helps, and don't really know the answer... sorry!

If any one would know for what L.J. stands, the answer would be easier.

Nick name?
Something similar as A.C.E?
...

As I don't know, I would have done the same as you did. 

I have no earthly idea what (l.J.) means, it doesn't match up with Union/Guild affiliation I am familiar with...but then it seems that other such whatever they ares have cropped up after this man's name elsewhere. So, because they are different, I think we can discount nicknames or some sort of intials pertaining to his name. The most likely would seem to be initials of whoever he was working for on a given film. But, I won't swear  to that, I would be more likely to swear at it. I think I would ignore it.

Ummm amusing bbbbb, b6t I doubt that sreiously. Initials can have many possible meetings, that is but one and has little do with the credit particularly when you uncover that he has another set of initials attached to another credit. BTW I presume you had your (tic)