Barts were tough-looking characters who sported haircuts like Elvis and wore motorcycle jackets and such.
Occasionally you'd hear about fights in Dolores Park, but a bad guy then would be armed with a chain, a knife at most.
You also heard about Pachucos (sp?) which, I gather, were a more dangerous group of Latinos. A popular tattoo then was a cross on the web of your hand between thumb and forefinger. I always thought of it as pachuco-esque sign.
There was gang in the Mission, during the WW I era, called the White Gang and I think they were a pretty bad group of guys.