The early Honda CX500s had head mounted crankcase breathers installed; later models in some areas had bottle breathers on the side of the motor, and a conversion kit was available to convert from the earlier to the later designs. Here’s one such kit.
Looking for a bottle breather yourself? CMSNL often have them in stock here.
Addendum 2017: Murray from Murray’s Carbs has let me know that the GL650 in North America had a similar kit – the rear pulser breather on that bike resulted in higher than acceptable oil consumption at sustained high speeds thanks to not being able to cope with the amount of windage from the motor and a similar kit was provided as a solution. It has a T-piece that allows a bottle breather to be plumbed into the hose coming from the rear pulser cover vent. Anyhow, back to this one – the instructions:

Routing of the new hose:

Kit contents:

These rubbers plug up the old head holes:

Requisite O-rings and clamp:

The bottle breather itself, part number 12361-415-610:



The new hose:
