What does the symbol of a crown mean on an old double barrel shotgun?

The crown is the proof stamp from the Birmingham proof house. All guns made in or imported to the UK were submitted for proof testing in either Birmingham or London. The stamp will have letters underneath it................'BV' denotes the 'viewing' mark, stamped while the barrels were in the white, before finishing. 'MP' denotes the definitive proof mark. These were applied to guns proved from 1904 to 1950, after which the crown was stamped atop 'NP'