Well now, the engine appears to be an A10 from 1954 but as you say the frame isn't an RGS. The GA 10 number is in the range of the last year for RGSs, 1963, but it clearly isn't. Maybe someone re-stamped the frame to try to make it seem like an RGS, numberwise anyway.
Where did you find the frame number marked? Around the same date as the engine it would be on the steering head facing forwards (me thinks).
(Just noticed your last photo, that's not where I would have expected to see the number stamped, well at least not on my frame)