[Clean Feed]

There's a bit about that here:

cleanfeed.thetvroom.com 

The streaking signal referred to in the article above was a test signal used to view and adjust ‘afterglow’ on a crt based telecine or slide machine. TCF was originated on such a machine (a Rank Cintel flying spot ‘jobby’ with a ‘sprod’ slide changing mechanism. It was not a test signal used to evaluate and adjust tv circuits. 

I wonder if the slide scanner was being checked and adjusted for afterglow correction infrequently at the time and was simply left to circuit whilst tweaking - instead of TCF. No matter.

I'm almost certain the short recording of the streak test has test card music playing in the background, so it's possible it was put to air accidentally.