Gun Collector's Digest #5 had an article on British commercial proof. Those long text of how many TONS etc on barrel was post 1955. The BNP stamp was introduced in 1955 as well. Before 1955, they used other text.
The 1955 rule is copied here:
-- NOT ENGLISH MAKE was eliminated
-- The viewmarks were eliminated
-- Birmingham proof and nitro proof combined into BNP
-- Caliber, cartridge case length, pressure added
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