Einstein signed the board at Manchester University where he delivered his first British lecture on the theory of relativity in 1921 and was made an Honorary Doctor of Science.
His signature on the bottom of the board was later cut out, framed and hung in the physics laboratory until 1960.
But when the laboratory was redecorated, the autograph was consigned to the cellar where, seven years later, it was rescued by Douglas Broadbent, a lecturer, just days before a junk clearance in which it would probably have been destroyed.
Mr Broadbent, now 75, kept the momento on a wall in his home in Wythenshawe, Manchester. He has now handed it back to the university to be displayed once again.
"I always regarded it as custodianship rather than ownership," Mr Broadbent said. "Now I am not so closely connected with the university it seemed the right time to return it."
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