In the movie Die Schweizermacher of 1978, this road sign is seen several times. However, I couldn't find any information about it.

The location from the movie is visible here, but the sign is no longer there: https://maps.app.goo.gl/QnmAz8zN6KfDnb8bA

Things I tried:

1. Swiss Road Signs from 1963 to 1979, not in there.
2. A military driver's handbook, doesn't contain the sign either, however it is a later version and I couldn't get the one which was valid in 1978. Anyway, would be very strange to see military signs in the middle of Zurich.
3. Asked my Swiss friends, they don't know it either.

There is a mention of a similar looking sign here, and this doesn't look like a reliable source for road signs either.

Can anybody elucidate the meaning of this sign in general and in context?