Hi Pigboat,
Thank you for digging that out. I've checked that particular Gipsy Moth and begin to see a little more clearly how the Moth floatplanes developed.
The first so equipped was DH60, G-EBOT which was fitted with floats developed for the Short S7 Mussel 1 aircraft. Subsequent Moths (both Gipsy and Tiger) appear to have had a standard fit of float which is very similar to the original Mussel design and likewise made by Shorts, as your letter confirms.
Shorts subsequently built the Short S7 Mussel II aircraft but it was scrapped in 1934. The amphibious float arrangement on this aircraft was used on DH60G G-AADV and also G-AAVC.
The only other Moth I know that had a central float was DH60M K2235. This had an experimental float which was a 1:24 scale model of the planing bottom of the Short Singapore II flying boat!
After all that it's frustrating that I can't find any plans or shots of the "standard" Moth float!