Great little edger Rod, I have one & seen a few of these, very well engineered.
I have found the patent for this edger 1939/40, one of the patent applicants being Ted Halford who founded HRG Engineering Co, in Tolworth, Surrey. They produced their own cars from 1936 to 1956. Halford & Co-founder Ron Godfrey had raced together at Brooklands and went on to produce HRG sports cars. In 1938 they had the highest placed British car at Le Mans and in both 1939 and 1949 won the 1.5 litre class. An HRG cost £395 (about half the cost of the 1.5-litre Aston Martin and weighed almost 1,000 pounds (450 kg) less.
This may well explain the excellent engineering of this little edger.
I'm not sure what name it had, or who retailed it, but I have found an ad in 1939 for a recently patented edge trimmer called 'The Snippet', said to be highly successful, priced at 19s 6d, could be the one?
Rod's edger is here https://www.oldlawnmowerclub.co.uk/club/events/rally-2020/rod-child/unknown-edger