Engine number question on thunderbolt

[huntingdon]

Hopefully somebody can help. I have thunderbolt from 1964 with engine number A65G753 does anyone know what original bike this came from as I am struggling to find a record of it. Hope someone out there can help.

[Bess]

H,
    Could it be A65C, rather than a G?

Best wishes...

[huntingdon]

Thanks
That was my first thought but it is definitely a G. Very strange.