In rec.radio.amateur.homebrew Gareth's Downstairs Computer wrote:
Comments have been that the Shortt clock is not
a phase locked loop because of the infrequent checking
but if you were to cast your mind back to the early
days of PLLs in VHF rigs, typically the 145MHz signal was
crystal mixed down to a few hundred kHz before being
phase checked, and you can't get more infrequent
than 1 in 1000, which is really slow compared to the 1 in 30
check rate of the Shortt clock!

That is because most people seem to be incapable of doing a simple web
search.

https://en.wikipedia.org/wiki/Shortt-Synchronome_clock

A Wikipedia article with 18 external references that says:

"This feedback loop functioned as an electromechanical version of a
phase-locked loop..."

http://www.meccanotec.com/shortt.html

Yet another article which goes into detail of the workings of the clock and
says:

"The slave is kept in synchrony with the master in a phase locked loop."


--
Jim Pennino