There is currently a gearbox on eBay under the heading "MG MMM PA Gearbox" . The seller says he cannot guarantee it is from a PA but it "looks correct" and the numbers are very visible.
As far as I know the number "75" on the casing suggests Wolseley Hornet (or at least a Wolseley Hornet casing). I have the impression 165 is the typical number on a PA gearbox and 199 on a PB.
Of course it is possible the '75' gearbox has been rebuilt to P-type specification.