My LM5 Deluxe has headstock truss rod access, but I haven't had to adjust the neck yet so I'm not sure on the size. I'm also not sure if it's the same as the 1600's (my neck is one piece maple so he may have had a different setup), but I'll check the size when I get home tonight with my allen wrench kit.
The access to my 2011 LM is at the top also and although I dont know the exact size, the allen wrench for adjusting my FBass BN5 works the truss on the bass. If its the same should be no problem finding the wrench then.

