Spoke with helpful gentlemen from Chelsea Clock - he remembered your tour.
Yes, Radium. As of December they are no longer allowed to service radium-bearing clocks (dials) due to lack of approved facilities. They use Geiger counters on all incoming clocks and claimed handling/contact was the main concern. I’ll start a separate ‘radium clock face thread’ if merited after researching more, but FYI for any old Chelsea radium clock owners wanting factory service.