1. How did you get this coin and who from
2. How much did you pay for it
3. I notice you are in Belarus, not a country that is typically known for its hotspot in buying scarce British coins.
4. Is there a certificate of authentication - or was one offered.
5. Is it magnetic?
If the answers are 1. From China, Russia, Eastern Europe or within the last 10 years - get real suspicious
2. Under $100 USD - Suspicious
3. Could be barking up the wrong tree - sorry but lets face it Belarusians are not usual members here.
4. This is a scarce coin worth several hundred dollars if real and usually any reputable dealer will give you some provenance if its real.
5. If it is - its fake.
Overall - to me the colour is wrong for this type, like NumisRobs they should be slightly yellowy silverish - yours has an iron like polished feel. But the weight is right.
The quarternary alloy was Silver 50%, Copper 40% and Zinc and Nickel 5% each - some minor variation is allowed as often the zinc, nickel and copper were melted together and the silver added later - so it would not 'pickle' the silver and keep it light.
The detail looks wrong - all this evidence to me suggests you have a high quality possibly Chinese lead or steel fake - that has been deliberately aged.
Loving Halfcrowns. British and Commonwealth coins 1750 - 1950 and anything Kiwi.
If it's round, shiny and silvery I will love it.