I'm wondering what other monstrous metric-English combinations are out
there?

There's a lot of specialty farm equipment where english nut and bolt wrench
sizes were speced. Not the bore and thread, so they are metric if that
component was built on the far side of the pond. Not a problem till you lose
a bolt or nut. Fastenal thought I was nuts, even they hadn't seen that.

My JD5310N has some of this, some pure metric, and some pure english. All
the fastener lengths are non standard. At least you can order replacement
fasteners from JD, just hold on to your shorts when you see the bill.

Karl