Mower is a belly mounted 60 inch mower mounted on a Mahindra emax 22. The mower is manufactured by Amerequip for Mahindra. Manual specifies three 21 inch blades; however, the aftermarket blades found on the mower measure 20-5/8 inches. I intend to replace; however, want to use appropriate aftermarket blades. Like 2x4 lumber, the 21 inch description might simply be a nominal reference, but, I want to be certain.
I don't know if they are "nominal" in the way of lumber, or just not that precise in the way of cheap lawn mower blades. The overlap between your blades will be reduced by 3/8 inches (Each blade is 3/16 inch short at each end). The overlap between the paths of the blades is typically closer to an inch so you'll still have plenty of overlap.