Awesome coin! I have a couple, one is the 53 and the other is a 66 proof with a different reverse. Fun fact is that Numista says the 66 proof balboa (90% silver) was struck at the US Mint, but the other proof coins were struck in both US or Royal Mint in London depending on the shape of the diamonds. It appears my set is US made because of the elongated diamonds (versus square diamonds) on the obverse separating the legend. So the US mint was striking 90% crown sized coins at a time when the US did not have any dollar size coins and was only striking half dollar US coins in 40% clad silver for domestic use. Sorry my picture of the '66 proof is in the OGP and hard to see the details.
That’s fairly graded, and a lovely example of the type. The reverse is particularly stunning in detail - and let’s be honest, the reverse is the selling point on this coin!