Seen from the one side....
...and from the other. The sling ring of course was added too.
The Mark 6 sopranos might have their strong points, but the position of the thumb rest is not one of them.
The thumb rest sits too low on the instrument, which makes it troublesome to hit the Tc and C3 (side-C and high-E). It's also quite an uncomfortable position for the thumb itself. A new rest solves both problems.