I think your scan is the best I've seen of the actual magazine cover, Dan, and perhaps I'll substitute it for the one I used on my NL page
back in 2004-05. [Note: after writing this, I did indeed substitute Dan's scan for the one I had been using on the page.]
I set up a side-by-side comparison of it and the scan of the original art, which is a little unfair of course, and I would have to say the original art scan is more evenly-toned although not more detailed than the cover scan. Because of modern technology, the limiting factor here would seem to be the process that reproduced the art on the magazine cover back in 1975 and not anything you did.
Lettick was indeed a very good artist (he may have spelled his first name "Birney" by the way - I found a painting attributed to an artist of that name with a signature that seems to match the one on the NL Cover). It's possible that he did
use live models, for the Lampoon's
100th anniversary issue, referring to the spanking cover, remarked that "conscientious P.J. [O'Rourke, NL's editor at the time] supervised every second of modeling." I wouldn't have minded taking on the Professor's role for the shoot, but I think I would have found it impossible to hold a slide rule over the female model's bare bottom for several hours without bringing it down smartly at least once