With such a diverse range of parts manufactured in house or by contract, keeping some common system of model identity for parts was a must. You will all be familiar with the two digit prefix for parts in the early days, 67 notably for our A rigids and plungers, 42 for later S/A variants, where newer or modified components were used. 68 for later unit models. Earlier parts used on later ranges still carry on with the original part number, so the system can appear confusing.
So I'd agree with CJ, but once fitted this marking could be ground off, and I don't think I have seen a frame with this mark remaining. As with anything, there are times in the industrial process were rogue bits get past the QC inspector and the norm may not be what you expect.
Came across a cut off frame stand lug with 90 prefix. This turned out to be from a Bantam, and was moved on to another happy sufferer......
A pity the existing frame has lost its taper fittings foot the footrests. I have one exactly the same, but view it as a challenge to improve the design and get some use rather than worry about trying to replicate the original fixings.
Swarfy.
