I have also seen a photograph of the C1W, float plane version, that has the same tail/fin structure as your C1 version.
Good morning Richard! Fascinating stuff, thanks for the additional information and yes, it is confusing, but interesting (to me). Another source (Gray/Thetford) shows a CII (three-bay), CIII, CIW and CIIW all with the fin shape shown in your silhouette but the CII photo has the fin obliterated, but I suspect it's the same machine you posted. I wish I knew more about WWI aircraft. Same book shows the CI with that odd looking circular fin.
G'day LM. Is it a homebuilt from say twenty years ago?
Cheers
Graeme.