I'd guess that 455 2nd Model acquired most or all of its British stamps before conversion to 45 ACP and the nickel finish.

I've seen the last 5 crossed out in the cal stamp but not as neat with an X. I wonder if the conversion was done in England by one of the shotgun makers like Cogswell. They always did nice work on Victory Model conversions; properly sleeving the chambers for 38 Spl, etc.

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Jim
S&WCA #819