I assume the L-bracket that came with your camera has an angled cold shoe on its top? Mamiya was making a round reflector for single-use flash bulbs, so that's where you should put it It's angled to prevent the bulky reflector from shadowing the WLF image. Many of old electroniс flashes did have a separate lamp unit and a generator/battery unit carried on a strap, connected with hi-voltage cables. So it's just not intended for usage with modern self-containing flashes, unfortunately I use my C330 with hammerhead Metz Mecablitz 202, put on its own bracket. That's a fine combination, though I am rather missing my pistol grip - it can't be used with this combo, unfortunately.

Cheers,
Zhenya