I now have the bicycle and it has been dismantled so that the frame can be carefully examined.
The seat stays are exactly identical in profile. The upper part of the stay above the bridge is straight. The lower part is a constant curve of 32" - approx. 810 mm +/-. There is no visible damage whatsoever of a rear end impact. The brazed joins at each end are not deformed in any way. At the top, the gold lug lining is still there, which indicates that either the frame was built up with the curved stays [possibly using a larger frame's stays] and the whole frame then chromium plated / Accrillicoated - or it was modified afterwards and the plating / painting etc repeated.
All the chromework had in the past been lightly greased.
There is a strange eliptical decal near the top of each stay, red with an elongated gold C and the whole surrounded with a pale blue "garland".
There is an X embossed on the underside of the bottom bracket and on the left rear drop-out the number 220107.
The top tube Carlton Cobra decal is in old script /Olympic rings and not the later "chubby" type, currently on offer on ebay.
The decals are similar in style to the 1952 Massed Start frame.
In my opinion, this frame was either a special order to Carlton from the original owner [he was the only owner before me- sold by his son] or equally likely, Carlton produced this frame as an experimental prototype for a possible future model and when Raleigh saw it, they weren't interested and somehow it was sold to the previous owner.
As far as I can ascertain the Cobra model was discontinued in the late 1960's, so it was only produced for a few years.
I stand corrected !