In reply to Mick Ward:
Many thanks to people for their various insights.
I must confess the whole business is a mystery to me. Clearly the diminutive entertainer wasn't noted for his grit routes. And I'm sure it's Jimmie, rather than Jimmy.
Someone suggested that Jimmie Clitheroe might be an affectionate(?) term for a then well-known climbing hero, seemingly usurped rather brutally by an aspirant. I wonder what happened to him. Did he, in turn, get usurped?
I'm wondering now if the year isn't 1973, rather than 1977. The writing's really dreadful. The forename on the signature might have an 'a' (or maybe an 'e'?) in it. The surname looks as though it could have an 'o' in it. But I can't be sure. And anyway, some things are better left unknown.
Mick