I agree with SC and pizzaboy. Love a lot of pre-1950 gangster flicks.

Some old classics you might enjoy:

"This Gun For Hire"...Alan Ladd
"Blast Of Silence"...(actually from 1960 I think)
"The Asphalt Jungle"...Sterling Hayden
"Out Of The Past"...Robert Mitchum and Kirk Douglas
Anything with Humphrey Bogart as a gangster...
This next one doesn't qualify cuz it was done in late 60's, but "Point Blank" with Lee Marvin is classic. Was remade as "Payback" with Mel Gibson.

Check any of those out if you're interested.
Also, what you have to realize is that movies made before Joe Valachi testified about the Mafia, no one in the general public, especially Hollywood, had any idea what the inner workings of LCN was all about.

"You don't make up for your sins in church. You do it in the
streets. You do it at home. The rest is bullshit and you know it."
----Mean Streets---

"Business bad? Fuck you, pay me. Oh, you had a fire? Fuck you
pay me. Place got hit by lightning huh? Fuck you, pay me. It was
beautiful!". ---Henry Hill in "Goodfellas"