500mm Lens Support Bracket Question

Hi,

I recently bought a Mamiya 500mm lens for the RB67 with a support bracket. The bracket has an unusually large threaded hole at the balance point, and another similar threaded hole about 6" towards the front of the lens. The camera shop didn't have a thread reducer and didn't know what type it was. It does seem that the front threaded hole already has a thread reducer in there which accepts a 3/8" bolt. I need to find another thread reducer to use at the screw hole at the balance point. Does anyone know the type required and where I can get one from? I tried taking out the thread reducer which is in the front screw hole, but it seems to be really and truly stuck in there.

Measuring with a micrometer it seems that the thread has an internal diameter of .42"

What is the screw at the front of the bracket actually used for?

Also there are two 'lugs' at the front of the support, what are they for?

My bracket is very similar to the one on this page, but not identical;

http://photo.net/medium-format-photography-forum/00YxeD

My bracket has a 'kink' at the end, like a small L-shape, but it has also has full loop at the end, like the one pictured on that page. It is branded Mamiya.

Many thanks,
Woodspiral.

Well, I've made some progress with this. I took the bracket to Nuts and Bolts on Clerkenwell Road in London and they found a bolt which seemed to fit and it was 12mm. Luckily I have an engineer friend who will be able to tap the bolt and make a reducer for me. I say 'seems' there as the thread is worn a little and the bolt didn't screw all the way in, so not 100% sure that 12mm is correct. 12mm is obviously not far off 1/2" but we tried that size and it was too big. Being japanese you might expect it to be imperial.

The two lugs at the front of the bracket fit the neck strap support, and they are round on the inside so allow full rotation. I notice that similar lugs on the front my RB67 Pro S camera have square insides which do not allow the strap connectors to rotate.

Even more progress - my engineer friend identified the thread and he said it was rather unusual. It is 12mm fine-thread, that is the pitch of the thread is 1mm (as opposed to normal 1.5mm). He has made me a custom reducer. Even armed with this information I was unable to find a reducer on the Internet, so it is very lucky that my friend knew what he was up against. The bolt that was sold to me by the Nut and Bolt shop was 1.5mm pitch so would never have fitted.