Line OutPut Transformer - used to generate the 25 or so KV from an overwind on said transformer using the horizontal flyback pulses to feed the eht rectifier valve or HV silicon diode / diode string (half wave rectification using the picture tube itself as the EHT smoothing capacitor).
If the LOPT driver circuit is powered from an insufficiently smoothed supply, the EHT will have this ripple superimposed upon it and show up as a variation in picture brightness ('Hum Bars').
Of course, you will see similar effects if the supply to the video amplifier channels is similarly afflicted. Indeed, it's likely to be the sum effect of these two sources of ripple interference by a badly filtered common source supply rail.
The early colour TV sets in the UK were hybrid valve/transistor designs where the penultimate LOPT driver valve was the last to be usurped by a very fast high voltage power transistor (the final irreplacable valve being the picture tube itself).
The low voltage psu for the transistor stages could easily be designed to eliminate mains ripple, leaving just the LOPT valve driver HT perhaps still relying on an older style less than perfect filtered supply.
I suppose the later hybrid designs could have made good use of high voltage power transistor to regulate the 300 odd volt HT supply and eliminate even this residual source of 'hum' (or perhaps, more likely now I think about it, an SMPSU was used where the only high voltage fast semiconductors required would be the fast switching rectifier diodes).