Thanks, all.
I downloaded the McMaster drawing. It was not an X-Acto blade, but an 'industry #11 blade', and NOT the same dimensions as the X-Acto.
Since every brand of 'industry #11 blade' I've purchased has been a different shape with a different lug hole position (or no lug hole, at all), I am forcing my customer to go with the authentic X-Acto #11 blades. Even their #11-M (which only offers a slightly rounded tip, instead of a needle-sharp point) is a different shape from the #11!!! The shape is different, and the 'lug hole' is in a different place in the shank.
I took the McMaster drawing into CB, and corrected the dimensions and various apexes' positions to exactly match the X-Acto measurements, then cut a recess in some Al stock that shape and 0.001" oversized to check fit. The blades fit closely and well, with no discernible play.
Lloyd