(There were also anarchists etc, but in general the non-communist extreme left-wingers were few before ~1968, when they got more numerous and varied).
Also, I remember reading surprise after 1989 on how much control Soviet had over the West European communist parties. E.g. that the Italian communist party had such an accomodating attitude by order (Soviet wanted to get a de facto split in Western/Eastern Europe.)
(But OK, even I know that Labor have changed a lot over the last 50-60 years.)
http://www.historylearningsite.co.uk/labour_party_nationalis...
Turns out that the NHS was a good idea and the nationalising of industries was a bad idea!
Of course, when Labour re-invented itself in the 1990s as "New Labour" they pretty much became "Conservative-Lite" - I'm still not sure what values they actually have.
The Swedish (ex-)communists became feminists and anti-racists (they argue for something called "structural racism" that native Swedes are guilty of to immigrants -- never with references to non-extremist sources).
In short, just the normal marxism, but another group than "workers" to create conflicts around.
This wouldn't be more than a detail, but the left wing extremists has a big, big representation in the journalist profession since the early 70s. You find lots of left wing academics saying weird stuff in the media, without being identified as e.g. extremist ex-mental patients (Hübinette) or anarchists which wrote admiringly about Ghadaffi (Gardell), etc.
Seing conspiracy theories like those, I am strengthened in the belief that the US extreme left and right are equally crazy. In Europe we mostly see the Chomsky brigade, some variation would make it less irritating.