In the Fifties the regulations stated that for a SCPL and a First Class ATPL you need, among other things, 500 hours in command time. In other words decision making time whether on multi or single engine types. There was no such animal as ICUS. Same with instrument flight time. It could only be logged when manually flying in IMC. Later that was watered down to automatic pilot monitoring in IMC.
Since then the requirement for 500 hours in command has been watered down beyond all recognition by reducing it to 100 hours command (?) and the rest made up of ICUS time. ICUS being, IMHO, just another word for pseudo dual hours with no responsibility since the real captain is watching you like a hawk
I wonder when that first watering down of the 500 command hours was first introduced and by whom and for what justification?