Hi Pigboat,
A very interesting photo indeed. The float is almost certainly the Shorts version as similar shots suggest what you have just stated - no water rudders. I would suggest that the floats did have a flute but this is hidden in the shadow.
The original installation of these floats was quite different and if you look at your shot again you will see two lines on the float which indicate the original strut attachment positions (although they are probably there for other reasons).
Do you have any photographs showing these floats in plan view?
I might just put together a web page of photo's just to show the confusing variety of float variations on Moths. The more I look around the more variety I see.