I have never heard of hydraulic brakes being fitted to one of our cars.
The original factory file for this car might give a clue. I wonder if a purchaser could have asked for his new car to be fitted with Wolsely Hornet brakes as a special order?
I guess you mean PA 1311, MMM 2600. There is no mention of hydraulics in my files, which include a digest of the chassis files - however this is incomplete, so the full files would need to be investigated to be certain!