91SR3C
Displacement : 14.88 cc
Bore: 　 27.0 mm
Stroke: 26.0 mm
Weight: 560g

91SR
Displacement : 14.88 cc
Bore: 　 27.0 mm
Stroke: 26.0 mm
Weight: 533g

The difference between 91SR3C and 91SR is

Almost same except engine head.
91SR3C is using more bigger engine head.
So, Better cooling for the engine.

However, 91SR is less weight.

Almost same except the weight and the cooling head.