E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A sixpénce. Called testone (teste, a head) because it was stamped on one side with the head of the reigning sovereign. Similarly, the head canopy of a bed is called its tester (Italian, testa; French, teste, tête). Copstick in Dutch means the same thing. Worth 12d. in the reign of Henry VIII., but 6d. in the reign of Elizabeth.
Hold, theres a tester for thee.Shakespeare: 2 Henry IV., iii. 2.
Testers are gone to Oxford, to study at Brazenose. When Henry VIII. debased the silver testers, the alloy broke out in red pimples through the silver, giving the royal likeness in the coin a blotchy appearance; hence the punning proverb.