In response to Howard and Chris, I've just had a look at my copy of a catalogue from around 1924/25 issued by W.G.Coram well known Douglas specialist and agent of Victoria Street, Bristol. Inside we have a picture exactly the same as on the box that Howard has with the following description;
'Terry's Spring Links for old pattern 2 3/4hp Douglas Forks, pre 1919. Replaces the top fork links. A wonderful shock absorbing device ... per pair 15/6'
W.G.Coram was responsible for the manufacture of many accessories for the Douglas such as the 'zoom zoom' exhaust for the 2 3/4 models and the 'zoom the second', a similar device made for the 4 hp models. Other items available were patent alloy air cooled valve caps, barrel type valve springs, Coram alloy pistons, patented twin rings for pistons, high duty valves, hot air intake pipe and even Silver Scot machine mascots. Even 'shop soiled' Douglas silencers taken from new machines when owners purchased a 'zoom zoom' silencer. Sold for half price at 15/-
According to the catalogue Mr. Coram was with Messrs. Douglas in charge of the Repair Department from 1910-1920. If I can get my head around the methods of sending photos/emails then I will copy some of the pages. They do make interesting reading.
Reg.