Well. at least now I've got a plausible source: 'English Dates in Dreadful Doggerel', an Instructional book written by A. G. Grenfell, with rhymes and short-cut methods of learning dates, monarchs and historical facts.
Grenfell was the headmaster of the former Mostyn House School in Parkgate, Cheshire, and a brother (I think) to Wilfred Grenfell, the medical missionary.
There's a copy in the Cheshire Archives, apparently - ref. SP 5/8427/46 but none on-line.