Here is a bit more info to add to to Dean's on the proof marks. The crown over R is for a reproof. The crown over STD shows a standard proof load was used. The 15.8 is the millimeters of the bore 23 centimeters from the breech. The 20 ga. ranged from 15.6 to 16.2 mm. The 20 in the diamond also stated the nominal ga. The LP over 07 is the year of proof. Hope that helps a bit.
Forgot to mention yours are London marks rather than Birmingham.
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