I was thinking we should be opening an Observer Corps thread!
I have a snippet of recent (1980s?) ROC rank, identical to RAF but all black, but later got a WW2 tunic only to find it's more of a dark blue and I think either slightly thinner and/or a different pattern. All my uniforms are in storage at the no but I'll see if I can add a photo sometime.
As to the Special Duties, I'm thinking the 'rank' was denoted by the style of badge, with perhaps later in the war a 'one rank fits all' style arrangement with the chrome badge superseding the cloth badges on the cap and cloth badges replacing the armlet. I'm pretty sure the photo of Sir George Edwards doesn't show rank and am pretty sure my picture of Pickard doesn't either so perhaps like Scripture Readers they had a status equivalent to say a Squadron Leader rank (SRs having equivalent status to Warrant Officers).
A very interesting topic.