When was the first ever rain race not to be red flagged?

[FIA]

When was the first ever rain race not to be red flagged?

[Rollo]

Monaco 1950 was a wet race, but I don't know whether or not it was raining. The Indy 500 for 1950 was red-flagged because of rain.
So I suppose that the first rain race was the Swiss GP at Bremgarten in 1951 and I think Fangio won that in an Alfa.

Perhaps someone on Forix or something should check this out, but I suspect that my nerdiness has reached new heights for even knowing this.