Do you mean a blanking circuit along these lines?
http://www.earlytelevision.org/vertical_blanking.html
I have added that type of circuit in a couple of my B/W sets (Admiral 19A12 and Philco 49-1240).
Out of curiosity, I walked around and turned on a few sets to see if all of the old ones show this interference. Here's the score card:
DuMont RA-103: no
DuMont RA-113: no
Philco 49-1240: yes (despite vertical blanking mod)
Philco Predicta Siesta H3412L: yes
Philco Miss America F4626M: very very faint
RCA CTC-7: yes
RCA CTC-11: yes
unknown 16" TV in Scott 800B combo: no
Now I'm at least as puzzled as before. The interference only shows up on certain cable channels. Some sets show it, and others don't.
Phil Nelson