Weird, although good news as that "L" suffix belt is easy to get. Even my local Mustang shop has them. :-)
However MARTI themselves supplied the "S" suffix belt for my 390GT and Cobranda's still sell them and listed them for my 68' 390GT.
As an aside I visited the house of the late Richard Smykay yesterday and (has several lovely S codes) who had amassed quite a stock of "correct" parts over the years.
He had 3 "S" suffixed belts on the shelf, ex MARTI's, and labelled for 390GT.
So who knows! Maybe the S belts were up to a certain date? Or maybe the present day "L" replicas only became available in the last year or 2?
Still I figure based on the current state of affairs, both are correct enough! :-)