I have been attempting to find some as well Two points: to be correct saddle springs should be in pairs LH and RH 'twists' and something a bit more obvious its the loaded position that matters.
Often springs are anchored on adjustable bolts they are on my Triumph Trophy and KSS Velo in that case simply by the sequence bolt head in spring, end of spring,nut tight, the thread as required, then two nuts each side of frame bracket and plate 31 shows something like that except spacers are shown rather than the 'middle' nuts which is a bit neater however on most of the works photos of single seat Douglas I note the springs look exactly correct and finding out that length is the puzzle as it may vary with model.