STANDARD MAINS MOVEMENT

Self starting mains electric movement 240v. 50Hz.

For both interior and exterior use (when hands are under glass).

For clocks up to 12 Inches (302mm) in diameter.

Supplied with hanging bracket, rubber washer, hand nut and centre fixing nut Ø 10mm.

Hand setting from rear.

Dimensions Ø 65mm
Depth 29mm
Height (excluding hanger 98mm)

Spindle lengths and diameters:
Minute length 12.5mm
Euroshaft 3.5 x 2.8mm
Hour length 8.00mm, Ø4.00mm

Second length 33mm, Ø1.40mm.

What is a Synchronous Movement?

A synchronous electric clock movement does not contain a timekeeping oscillator such as a pendulum, balance wheel or quartz crystal, but instead counts the oscillations of the mains current from its wall plug to keep time.
Using mains frequency is a excellent way to keep time as, though there are minor fluctuation, electricity supply is legally required to average out at 50hz.
Full explanation from the National Grid here.