A question of dates

The dates given for Kover in "Watchmakers and Clockmakers of the World" are ca.1720-ca.1760 and the reference cited is the Dennison Watch Collection. The catalogue of the 1913 Exhibition of the Dennison Collection gives the date of the Kover watch as 1700.

I'm wondering what was the source of the date range ca.1720-ca.1760. I've Googled for any clues but have drawn a blank. Any suggestions gratefully received.

So far I have collected the following dating information:

From Watch-wiki...
Kover, London, Work No. 2570, 50 mm, 99 gr., Circa 1759
Silver double-spindle housing pocket with tortoiseshell-over housing and decorative nail decoration

From Old Bailey Proceedings, 16th September 1761...
A watch marked with the name Kover and the number 9430 was stolen in 1761

From the account book of Thomas Tye 1713-1783...
"1 November 1752 Exchanged a Silver Watch with Mr John Colton in Turncalf Aley for a pinch Beack watch Double Gilt with Gold a new one he brote with him from London No.8990 makers name Kover London gave in Exchange as by his receipt for the same £3.0s 0d"