Victoria Gothic, Quartered Arms, 10oz Coin

I have a 10oz coin with the gothic portrait of Victoria on one side and the quartered arms on the other. It is gold, and says 999 but I'm not sure if it is pure gold or plated. There is a tiny marking that looks like it says PD or PB. Someone told me there is another metal underneath and someone else told me it might be a medallion rather than a coin. I can't find any online that aren't just a single metal. 

Does anyone know what it could be?

https://britanniacoincompany.com/buy-coins/silver-coins/2021-gothic-crown-reverse-10-silver/

https://britanniacoincompany.com/buy-coins/gold-coins/2021-gothic-crown-reverse-10-gold/

It might be either of these, but my guess is that it's likely a silver one (if it is real)

Both versions are 65mm in diametre, does your one match?

Thanks very much for your reply.

Are there fakes of these coins going around? It is a bit larger than 65mm. A lady from chards said there are no hallmarks on the rim of the coins and that it might be a medallion. I can't find any medallions online. The hallmarks don't seem to tell me what metal it is if there is something underlying the gold. Have you seen any silver with gold?

It is a medallion. It's not a Royal Mint issue as they don't have mint marks for purity. 

Thank you for confirming this Peter. I haven't seen any of that particular medallion online. Do you know if they ever make them with palladium? 

Why do you think it's gold? The color definitely is not that of gold at least no .999 pure gold.

Also the hall marks say it's silver and the assay office is London after 1974.

Edit: hall mark chart (open in another tab to expand)

Thanks for replying. Yes I knew it couldn't be solid gold because I'm not that lucky..someone has x-rayed it and told us another metal is underneath. It was the shape of the 999 box that was throwing me off, it seemed halfway between square and oval. Wishful thinking maybe.

The colour in real life is quite yellow but I thought the tiny hallmark could have told what metal for certainty.  Your chart is extremely helpful 

I've seen them pop up on Facebook every so often. Not any on ebay right now. It's a great design hence been copied so much.