I believe you are mistaken but a lot of people are. This guy sells 6 types of 7/8th bar master cylinders. None are for the cx/gl/cb or what ever other motorcycle anyone has. What the seller does is list the MC that is closest to the standard for that type of motorcycle. So the one that is listed for the gl500 is listed for every two piston dual caliper bike. What he does is takes that and says someone has to be looking for one mc for a gl500 and then people looking for a gl500 MC see it and fight over it. Where as if he said this is good for all then there might be lest fighting from people who think that it has to be specific.
Just want you to be safe. Sure the one you listed may be good if you have a single caliper bike Most mc's are 12-16mm bore depending on where they fit on the chart.