I couldn't find any photos of Lightnings with the rocket trees but the written evidence strongly suggests the they are correct for L, M and some J models.
According to
http://www.kazoku.org/xp-38n/articles/p38info.htm:
"The L and M were produced equipped with Christmas-tree style rocket launchers. Some Js and earlier models were retrofitted with the Christmas-tree launcher. Bazooka-style rocket launchers (triple-tube cluster) were also known to have been fitted to the fuselage."
Another site notes that the tree launchers were fitted because wing mounts failed (
http://www.aerofiles.com/JBlock-p38j.html):
"The 2,520 P-38L-5-LOs had submerged fuel pumps and, after the unsatisfactory testing of 14 five-inch HVARs on zero-length launchers beneath the outer wing panel, underwing rocket "trees" for 10 rockets were mounted. "
And here is a photo from the P-38 manual showing how the tree was assembled. I'd say if it was in the manual it was probably fitted.
