It says Mitchell Brothers LTD on the rear fender, what do you think?

@@mikeskidmore6754 That decal on the rear mudguard, Mitchenhall Bros AVON tyres are a waterslide sticker that is available reproduction as are those tank decals. All genuine Rickman Metisse's have a serial number stamped on the headstem & that number is listed & can be found in the Rickman book. In the quick glimpse I caught of that frame number, it didn't look like a Rickman or vintage number. If it was a genuine Rickman Metisse, vintage or otherwise as you can still buy them now from two seperate companies like Rickman or Metisse, the seller would have said so as it adds value.

Left side pipe is obscuring engine no. but that's a black cast iron barrel.