I'm 65 years old.
The only memory of a 1950s test pattern, other than Testcard C of course, was the black cross on a white background which I know from Alan Pemberton excellent work
https://www.radios-tv.co.uk/Pembers/...rds/index.html
was known as the cruciform or 'art bars', art for artificial, because it was generated electronically.
If my childhood memories are correct, the cruciform was always accompanied by a tone which was louder and of higher pitch than the tone that occasionally accompanied Testcard C.