Originally Posted by Cattle Airlines

Not a question, but a statement.

The DC-9-21 was a rare variant, only 10 -20 series made. They had -30 wings but a short -10 style fuselage. Only Scandinavian ordered them.

There have been a number of rare variants.

Qantas had some rare 707s. Not many airlines had the 737-100 or 737-600. The 737-200 Combi is not that common. The 757-200 Combi is downright rare, with only 2 made for Royal Nepal. However, the engineering to design the cargo door was reused to design frieghters for UPS and others. Any variant of the Convair 880 and 990 are not that common. F28-1000 are not that common. VFW-Fokker 614, sort of a RJ, is extremely rare. So rare that VFW bought back planes to save money on producing spare parts. The Dassault Mercure also was only sold to Air Inter, selling 11.