The Roman Harvest pens use a slightly different set of bushings to the Cambridge Bushings.
1. As noted by others, the tubes walls are slightly thicker but the outside finished diameters are the same. This results in the internal diameter being slightly smaller.
2. I have not gone down the path of forcing people to use 2 separate diameter mandrels. All my bushings fit on the standard 1/4" mandrel. You will not need to purchase a "B Type" mandrel.
If you want to use the Cambridge bushings, don't thin the walls down the walls of the tube. If you do then you will end up with the parts fitting loosely onto the tube, use Epoxy or thick CA to hold the pieces.
You can reduce the outside diameter of the part that goes into the tube of Cambridge bushings With a HSS tool or carbide tip tool enough so that it can fit into the tube.
Regards
Timberbits