Stock footage of the Sandpiper Cessna featured three different planes. The numbers were N121PP or "November 121 Papa Papa'"(the one always given by Joe or Brian over the radio), N121PB and N160PB. The latter two are never spoken of.
Oh Lowell, please tell me your brother didn't marry his sister.
Roy Biggins: No. Cousin. The sister didn't work out.
In episode 4.2: "The Fortune Cookie", Joe decides to dissolve Sandpiper and Lowell fears he's out of a job, but in episode 3.14: "Stew in a Stew", Joe clearly states that Lowell works for the airport, not Sandpiper, meaning he still has the six planes at Aeromass to maintain.