I might take the “world’s first helicopter assault” claim with a grain of salt.

My guess would be the US in Korea with H-19s or to be precise, HRS (the basis for the Whirlwind) when the Marines of HMR-161 staged an assault on 21 Sept. 1951. A few days later, it staged a night assault, tricky stuff in the pre-nvg days.
Before then, smaller helicopters H-13 and H-5/HO3S (the basis for the Dragonfly/Widgeon) in the UN action were used in their more traditional roles as observation and liaison, and of course casualty evacuation.

Also in 1956, the French used the Piasecki H-21 (father of the Chinook) in Algeria.

It may come down to how one defines “assault”.
In addition to the Marine action, know the USAF H-19s inserted special squads in Korea for recon work as well as recovering a wrecked MiG-15, as well as the well documented rescue work.

Still, it sounds like it was interesting. More notice needs to be made of rotary wing history.

For more information, read Whirlybirds, U.S. Marine Helicopters in Korea, by Lt. Col. Ronald J. Brown, available free by doing a Google search.