Please help identify square coin

Hi,

I hope someone can help identify what this is - even if it's actually a coin or not!

It was included in a collection of coins I inherited, which are mostly UK 12th - 18th century, however this is very different.

It's 18mm x 18mm, weighs 7 grams and is impressed on only one side.

Does this look like a coin? Any suggestions as to what it might be?

Thanks in advance.