Leigh dovetail jigs use a combination of a dovetail or straight bit with a guide bush, rather than a router bit with a bearing.
The Leigh guide bushes have an elliptical shape to them, which gives adjustability when the router bit is sharpened, and the diameter of the bit has been reduced, rotate the guide bush to compensate when doing finger joints.
Generally this feature is only used for finger joints, as it is easy to use the jig to make the required adjustments for the other joints.
The three sizes of Leigh elliptical guide bushes are e7 7/16” (11.1mm), e8 1/2” (12.7mm) OD and e10 5/8” (15.87mm) OD.
The e10 Guide bush is standard with the RTJ400 jig and R9. The e10 guide bush has an inside diameter larger than 1/2” so allows for larger shanked router bits to be used, therefore longer depths of cuts can be achieved.