[ceebee23]

The Coronation took place in June 1953 ... so no NTSC standard let alone a 405 line version was in place.
The experimental system was apparently a field sequential system.

The set is almost certainly a projection set or the image is a slide lit internally from behind.

The later 405 line NTSC tests came about as a result of pressure from ITV which wanted colour for commercial tv. Sir Lew Grade apparently believed colour would mean advertisers would pay more.

The BBC was not too keen but did carry out tests but was more interested in waiting for the 625 line system which first broadcast on UHF in 1962 as BBC2.

Colour arriving after a long debate, using PAL, on July 1 1967 for Wimbledon, although tests had been carried out in 1966.

Ironically 405 line NTSC and 625 PAL would have looked pretty similar given the actual displays ie resolved lines on the sets of the day.

Which was part of the reason BBC did not want 405 line colour.