Do you mean the one that fits on the rear subframe on early cars?

They appear to have been out of production for a while. I was after one a
year back to turn into an adjustable valve for my kit car as per
http://hometown.aol.com/dvandrews/bpvalve.htm. and I couldn't get hold of
one.

Take a look at Keith Calvers article
http://www.gr8website2.com/calverst/articles/brakes-rear_brake_bias_adjustment.htm
as this may offer up some alternatives. How about using one of the later
bulkhead mounted items? Or converting your old item into the MS72 adjustable
type?

Peter