It was done by a very clever man," said Mr.Flexen.
"A first-class young scoundrel," Mr.Carrington amended.
"We shall soon see," said Mr.Harrison, politely incredulous.
The clerk came with the letters.

There were eight of them, all written by Mr.Manley and signed by Lord Loudwater.
The manager compared the signatures of every one of them with the signature in question, using a magnifying glass which lay on his desk.
Then, triumphant in his turn, he said curtly: "It's no forgery." "Allow me," said Mr.Flexen, and in his turn he compared the signatures, again every one of them.
Then he said: "As I said, it's an uncommonly good forgery.

You see that the bodies of the letters are all written with the same pen, a gold-nibbed fountain-pen; the signatures are written with a steel nib.

It cuts deeper into the paper, and the ink doesn't flow off it so evenly.
The forged signature is written with the same kind of nib as the genuine ones.

Also, the bodies of the letters are written in a fountain-pen ink--the 'Swan,' I think.