That may well be the case, but I wasn't around in 1898, and nor do I shoot blackpowder loads for upland hunting. I certainly wouldn't use #10 shot today for grouse. I want to be sure they are dead when they hit the ground or die soon after. In that regard I find 6's do the job just fine.
The issue I'd have with 6's for grouse: Not necessarily a problem if you're not also shooting woodcock. Especially with standard small bore shot charges, 6's would produce a pattern that could well lack sufficient density for woodcock. I'm pretty sure most hunters would choose something between 6's and 10's for mixed bag grouse and woodcock hunting. Likewise pretty sure that either 7 1/2's or 8's would be the most popular choices.
