The 523 is a 3XL version of the 516. To compensate for the additional leverage generated from the extended length Firehole Outdoors increased the wire diameter by one size, e.g. 523 #10 and 516 #8 have the same wire diameter.
This is a 3X long length hook with a 1X gape.
It has a 60 degree down eye and is formed from a heavy weight wire.