Stays of this type have been attributed to the framebuilder Tom Board. I have a 1967 Geoffrey Butler built by Cliff Shrubb with this type of stay, unlike the"shot-in"type sometimes used by the SBDU.
SBDU founded in 1974 at Ilkeston, and Carlton bought by Raleigh at the suggestion of a certain Mr Reg Harris.
Nice machine although a few hands too tall for me!
Roadking.