It's not really metrication, is it ? I ought to be a tad more precise at times .. have to blame the advancing decrepitude, I guess ...
(a)
current rule
(b)
old FAR 23 rule which changed to 61 kt at A/L 23-7 Sep69
(c)
CAR 3 goes back to 1949 (refer 3.83). I don't have any Net references to earlier US regs but, no doubt, someone else in the sandpit will be able to cite a link to the real olden days .. 1949 - I was too young to have much of an interest in aeroplanes ..
Here endeth the useless bit of information for the day ...