OK, after looking at a few more Weaver documents, the database I was looking at did not contain the PN #1230 (ribbed side with finned beaver-tail) which, along with the PN #1231 (smooth side with classic beavertail) was available in 2001. From this additional data, I was able to discern that the -S/L/LP stood for 2-rail, 3-rail without sound, and 3-rail with sound, respectively. Of course, the LP wasn't used with cars or car sets.
I also noticed that some of the Hiawatha cars were available as separate sale cars during same time period as PN #1613 through #1616.
Finally, this document shows several engines, including both Hiawatha types (4-4-2 & 4-6-4), as newly featuring Railsounds and TMCC in 2001. Hope this helps.