A roll pin technically is made of steel rolled round and round like a tootsie roll and does not have a gap. A split pin is just that, rolled nearly all the way around with a split (or gap) in it.
AR15/M16 etc ejectors have split pins in them, the tension outwards in the hole they are in holds them in. I have never seen anything else but a SPLIT pin in the numerous guns I have seen including Colt, Diemaco (Colt Canada), Bushmaster etc.