What were they

"What were they?" asked Holmes eagerly.
"Well, I first had the hammer examined. There was Dr. Wood there to help me. We found no signs of violence upon it. I was hoping that if Mr. Douglas defended himself with the hammer, he might have left his mark upon the murderer before he dropped it on the mat. But there was no stain."
"That, of course, proves nothing at all," remarked Inspector MacDonald. "There has been many a hammer murder and no trace on the hammer."
"Quite so. It doesn't prove it wasn't used. But there might have been stains, and that would have helped us. As a matter of fact there were none. Then I examined the gun. They were buckshot cartridges, and, as Sergeant Wilson pointed out, the triggers were wired together so that, if you pulled on the hinder one, both barrels were discharged. Whoever fixed that up had made up his mind that he was going to take no chances of missing his man. The sawed gun was not more than two foot long--one could carry it easily under one's coat. There was no complete maker's name; but the printed letters P-E-N were on the fluting between the barrels, and the rest of the name had been cut off by the saw."
"A big P with a flourish above it, E and N smaller?" asked Holmes.
"Exactly."
"Pennsylvania Small Arms Company--well-known American firm," said Holmes.
White Mason gazed at my friend as the little village practitioner looks at the Harley Street specialist who by a word can solve the difficulties that perplex him.
"That is very helpful, Mr. Holmes. No doubt you are right. Wonderful! Wonderful! Do you carry the names of all the gun makers in the world in your memory?"
Holmes dismissed the subject with a wave.
"No doubt it is an American shotgun," White Mason continued. "I seem to have read that a sawed-off shotgun is a weapon used in some parts of America. Apart from the name upon the barrel, the idea had occurred to me. There is some evidence then, that this man who entered the house and killed its master was an American."
MacDonald shook his head. "Man, you are surely travelling overfast," said he. "I have heard no evidence yet that any stranger was ever in the house at all."
"The open window, the blood on the sill, the queer card, the marks of boots in the corner, the gun!"
"Nothing there that could not have been arranged. Mr. Douglas was an American, or had lived long in America. So had Mr. Barker. You don't need to import an American from outside in order to account for American doings."
"Ames, the butler--"
"What about him? Is he reliable?"
"Ten years with Sir Charles Chandos--as solid as a rock. He has been with Douglas ever since he took the Manor House five years ago. He has never seen a gun of this sort in the house."
"The gun was made to conceal. That's why the barrels were sawed. It would fit into any box. How could he swear there was no such gun in the house?"
"Well, anyhow, he had never seen one."
MacDonald shook his obstinate Scotch head. "I'm not convinced yet that there was ever anyone in the house," said he. "I'm asking you to conseedar" (his accent became more Aberdonian as he lost himself in his argument) "I'm asking you to conseedar what it involves if you suppose that this gun was ever brought into the house, and that all these strange things were done by a person from outside. Oh, man, it's just inconceivable! It's clean against common sense! I put it to you, Mr. Holmes, judging it by what we have heard."