Going from a 1950s period, the options are a bit more limited, but there are still a few very good, very distinct designs from the period:
The Hughes OH-6 Cayuse is in many ways the predecessor to the more modern Little Bird. It has a distinct bubble canopy and a compact shape and is one of the earliest attack helicopters put in service.
The Bell UH-1 generally had side mounted miniguns / machineguns.
The Aerospatiale Alouette II exudes classic early helicopter design. Winglets could be added as a weapons platform.
If we drop period look for the helicopter, we have a few more pretty influential and recognizable options:
The AH-6 Little Bird, the successor to the OH-6 has secured it's role as a fast attack and transport helicopter. (Though this still fits the look of the OH-6, and the period, very well)
The AH-1 Cobra first saw service during the Vietnam war and is one of the most recognizable attack helicopters.
The AH-64 Apache is the most modern attack helicopter in wide-deployment.