So I picked up a new mower blade that sported a sticker claiming it probably fit my Craftsman mower. Got it home, took off the old blade, and it actually fit the mower; the holes matched the hub’s drive pins, although the bolt hole was oversized.
The old blade was a replacement, too, with a square hub hole and an adapter to fit the bolt. The blade had slots for the drive pins, so the adapter was required.
Seeing as how nothing exceeds like excess, I rummaged around in the heap to find something that would serve as an adapter in the central hole. It’s not really necessary, but I’m that type of guy.
As it turned out, an ordinary lockwasher for a 3/8 inch bolt was just about perfect. I crunched one in a short bolt with two nuts jammed in place …
… introduced it to the coarse side of Mr Grinding Wheel, and, after a few shots with a hammer, it became a perfect fit:
Bolted it on the mower, put in two hours of yard aerobics, and it worked just fine. Sliced the top off a root that evaded the attention of the previous blade, too.