The -M option specifies a reflection in a plane passing through the origin.
To reflect in a plane that doesn't pass through the origin: translate by
the negative of the coordinates of a point on the plane, then mirror
in the required direction, then
translate by the coordinates of the point. In this example the model
is reflected in a plane through the point (0.2,0,0).
View this model with the command
off_color -r A0.2,f ant4 > orig.off
off_trans -T -0.2,0,0 -M 1,0,0.0001 -T 0.2,0,0 ant4 | antiview -C 0,0,0 - orig.off