Dr Trevelyan tells Homes that every day Blessington would check the books and give Trevelyan 5 shillings and 6 pence for every guinea earned. Since the arrangement was that Blessington would earn 75% of the earnings, and a guinea is 21 shillings, he should have given Trevelyan 5 shillings and 3 pence.
The typeface used for the 2 signs outside of Dr Trevelyan's office ("DR P. TREVELYAN" and "KNOCK") is ITC Benguiat, which was released in 1977.