Hi Kye Fantastic! Many thanks for your extensive work in recording the little Simplex, an Australian-made engine used on many OZ lawnmowers.
ODK members should appreciate your effort in making sense of these old engines - where little factory records survive.
There is certainly a pattern - of sorts. This does not mean that the alpha-numeric stampings will reveal more; beyond smaller numbers meaning an earlier production engine date.
Much may be internal factory logic and only understood via Hardman & Hall documents (that we have not found).
Speculation is easy.
Your work reminds me of the classic story of persistence and research: that of Henry Ellis and his efforts to identify info. in the great British J&P lawnmowers. His story, published in three editions of the Old Lawnmower Club'sGrassbox magazine remains an inspiration to me.
Henry gathered heaps of serial markings before he made a breakthrough. I know you have read this story - taken from Grassbox 44, 48 and 50. I reproduce these articles below - as they are a great read ... and may be of interest to members.
