Flake said:
Canons system attaches via the lens hood (on compatable lenses) the adaptor gives a replica of the lens hood fixing on the lens. Third party systems use the filter thread directly...
No, they do not.
On directly compatible lenses (50mm f/2.5 Compact Macro, MP-E 65mm, 100mm f/2.8 Macro USM), the Canon macro flashes attach via a ~58mm diamater groove around the end of the barrel, which is distinct from the hood mount (when present) and filter threads.
The 50mm f/2.5 Compact Macro does not take a hood at all (the front element is recessed), and that lens has 52mm filter threads.
The MP-E 65mm's lens hood (a short, conical hood that most people don't even know exists) threads onto the lens' 58mm filter threads, and does not interfere with the flash attachment. Screwing a filter onto the MP-E 65mm does prevent the flash from being attached, because the flash mount ring cannot be pushed onto the lens far enough to reach the groove (and for me, since I use a filter on all my lenses, my solution is to mount the filter, then the Macrolite Adapter 58C, then the flash; the filter could also be screwed onto the threads on the flash mount ring itself, but too much distance between the filter and front element is bad).
The 100mm f/2.8 (non-L) has bayonet hood mount behind the groove for the flash, meaning flash and hood cannot both be attached. Like the MP-E 65mm, a filter prevents the mount ring from accessing the groove.
The Canon macrolite adapters replicate that groove for lenses that aren't directly compatible (Macrolite 67C for the 100mm L Macro, Macrolite 72C for the 180mm L Macro, and Macrolite 58C for the early PowerShot G series and the PowerShot Pro1).
If 3rd party flashes use 58mm filter threads, the solution for the 100mm L Macro would be a 67→58mm step-down ring - and since that would come from a 3rd party too (e.g. General Brand), it would be a much cheaper solution than the overpriced Canon adapter.