I did a little online searching and happened to find a page with a description of the coin.
Diameter: 38.61 mms
Weight: 22.62 grams
Silver: .3636 troy ounces
The site says it's five shillings, And that it was made before they used a formal decimal system on it's currency, interesting how they would make a coin without any indication of it's value. (That must have been quite confusing. )