"It might be that 'testing' downstream of the forcing cone is intended to exceed stated proof pressure at 17 and 25mm's."

It depends what you mean by that.

If you assert that proof chamber pressure is being applied to the 162mm point, no. Each barrel would blow up.

If you mean they are using overall hotter proof loads to assure the 162mm test spec is being met, perhaps.

The rules of proof are fond of the term 'at least' so you really don't know what The Masters intend to do to your gun, do you?

"The price of good shotgunnery is constant practice" - Fred Kimble