If you look here:
link ...
on my post #93 you will see the side by side pictures. Not only is the new reflector longer, but also wider. If you swaped the reflector from a recent 3W BOG module and screw it into the 5W module, then it of course fits - Flavio did that I I have done this myself as well. Basically, the two LED/driver "modules" are identical - it is the reflector in the 5W what's the "problem".
I guess you can file the 5W reflector down or cut it somehow in order to fit a standard "C" series head, but you would have to be carefull not to damage the reflector's finish.
Will