Modern day #9's if I recall right number to about 585/oz. the largest #10's in my 1890's chart go 822/oz. now that is quite a difference from the #9's. 5/8oz x 822 = 514 shot. That's equivelent to a 7/8 oz load of #9's. I simply cannot imagine taking such a load out after grouse. I never go smaller than #8 (4410/oz) for quail or dove, personally & usually use #7½. From articles I have read #10's used to be quite popular in small gauge, light loads for rail shooting, but the rail is a very small bird in comparsion to a grouse.
From reading old articles it seems many had the idea small shot slipped through feathers better than large shot. Perhaps this hunter had a similar false belief.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra