[2][3] The reason for this is that the meaning of the phrase per procurationem is ambiguous if used with undeclinable English names.
The other option is that "per procurationem" could be understood as a complete adverbial phrase in itself — "by the agency [of another]", without a dependent genitive.
A common usage of per procurationem in the English-speaking world occurs in business letters, which are often signed on behalf of another person.
(per procura autoritate) indicates that the person signing the document has special authority according to commercial law to sign documents in the name and on behalf of the company that a normal employee or representative of the company does not have.
Procuration originally took the form of meat, drink, provender, and other accommodation, but was gradually changed to a sum of money.