Hi,
My guess is;
A skilled engineer in the 30's, let's call him Bob, interested in model hydroplanes, saw one of the stuart turner engines in one of his contemporaries boats- this man was a big-head and was very proud of his acquisition and machining skills- it being 'the best on the market' and such (i'm sure we all know one!).
Our Bob politely agreed and looked into getting one for himself- on receipt of the castings he was very disappointed to find that the castings were in fact CI not Ali', after all, he wanted the edge on his freind with the 'best one'.
So, using the original castings as patterns he cast his own castings in alloy, however, due to shrinkage and not having a sophisticated enough casting set, he couldn't cast the valve-head- no matter what he did he would cock it up every time.
'Sod this!' sais Bob, i'll make one to my own pattern- he re-cast the crank-case with bearing houses for a vertical layshaft and fabricated his own valve-head using MS.
When, on taking it down to the pond for the first time, he showed his freind, he asked Bob where he purchased such an interesting engine?
"Oh this" said Bob "I made it myself, Aluminium you know, better than anything on the market."
Then proceeded to trounce said freind in a round the pole race- leaving him wondering why the advert said his engine was the best!
Thats what I think anyway!

Greg