From
Fuselage - ATA53
AFT BODY VORTEX GENERATORS
There are four vortex generators on each side of the rear fuselage above the horizontal stabiliser. The 737-1/200s also had three vortex generators below each stabiliser. They were probably installed to energise the airflow at the stagnation point at the tailcone, thereby reducing drag and giving a slight performance advantage.
Classics were initially produced without any aft body vortex generators (see photo. However the upper vortex generators were reinstated after line number 2277 (May 1992 onwards). This was to reduce elevator and elevator tab vibration during flight to increase their hinge bearing service life.
The CDL says that if any of these vortex generators are not fitted or missing “occasional vertical motions may be felt which appear to be light turbulence These motions are characteristic of this airplane and should not be construed to be associated with Mach buffet.”