What Charles didn't say, although you might have guessed, anyway, is that the line conversion was modified to produce a shorter line (both 525 and 625-line lengths are virtually identical) so that the correct aspect ratio was maintained.
Ah, that explains it - I worked on them in about '68/'69 IIRC.
And of course it is much less intrusive than the directed advertising on the Internet by Google et al. When we are not in a position to regulate internet data collection and personalised advertising it would be pretty pointless to interfere with broadcast TV.
Ha :-)
Thought I was rather too well pissed up first time I saw them :-)
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