[Bluecortina]

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.