Max: I hate to disagree with you, but the shim is for mounting at 0.293! A shim on top of the box would raise the mounting height, or, alternately, lower the coupler.
I'm sitting here, with a box of 1015 couplers in front of me, and the back shows "Fig A with shim, .293 (7.4 mm)" and "Fig B without shim, .279 (7.1 mm)".