I'm guessing the black paint looking coating is Suncorite, also known as "stoving". It was used during FTR (Factory Thorough Repair) on small arms throughout the British empire, usually applied over a phosphate finish.
Can you tell whether some of those areas where the coating has worn off look like phosphate/Parkerizing? It could also be applied directly on top of whatever bluing was present at the time but again, usually over phosphate I think.
-Bob
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