737 SP/SL?

From:         "Robert M. Sherry" <sherryr@a.cs.okstate.edu>
Organization: Oklahoma State University, Computer Science
Date:         10 Jun 96 12:22:20 
View raw article
  or MIME structure

Can someone tell me what a 737 SP is? or a 737 SL?
I have two photos, one of an Air Nauru 737-200
weearing "Boeing 737 SP" titles on the nacelles,
the other of an Air Berlin 737-300 with "Boeing
737-300 SL" on the rear fuselage. Are these real
variants of the plane, or just an individual
airline's gimmick?