Self-guiding cutters for rounding over edges do not require a side fence.
They are supplied with two bearings.
Use the larger bearing for a round-over.
If the cutter is lowered a top quirk can be produced.
A full round/bullnose for stair treads can also be produced by taking a second pass from the opposite side of the workpiece.

Specification:
D=38 mm
C=19 mm
B=9.5&12.7 mm
R=12.7 mm
OL=67 mm
Shank Diameter=1/2 inch