All these laserdiscs are recorded in CLV format. That's the long-play version which shows up as minutes and seconds, not frames, on the laserdisc player. In CLV, you get get just over an hour on each side. The other format, CAV, only fits 36 minutes or 54,000 frames per side so to fit a full 4-part Dr. Who on one disc, CLV is used.

Hm? Oh, you're still puzzling over that spelling error.

It's "principal", which was spelt "principle" on "Day of the Daleks".

It could be wrong on "The Ark in Space" for all I know, I've realised I haven't got the inlay sheet, have I?