Didn’t he take his honeymoon of at least one of his marriages in the USSR?
That doesn’t sound like a Fabian socialist to me, sounds like a Marxist to Stalinist socialist.
Maybe he has “evolved” since then?
I’ve heard he also flew red flag once in front of the capitol of the town he was Mayor in.
He was part of a Mayoral delegation to the USSR the day after his marriage, took his new wife and ironically called it their honeymoon. Below is a very complete story of his life. He actually had several favorable votes, from the FR perspective, on gun issues.
https://en.wikipedia.org/wiki/Bernie_Sanders