Its an L code meaning it came from the factory as 383 -2bbl.
If it were N code, it would have come from the factory as a HP383 4bbl, which has slightly (dont remember how much) more HP than the 383-2bbl..
That being said, I dont think the N code engine was available on challenger verts that year, as it was only available with the R/T challenger, and there were no R/T verts in '71.