From: The Newer Methods of Blood and Urine Chemistry, by  By Rutherford Birchard Hayes Gradwohl, Abraham Jacob Blaivas, 1920, Page 369.

“The mouthpiece is made of soft pure gum rubber, and consists of an elliptical rubber flange having a hole in the center 2 cm. in diameter, to which on one side a short rubber tube is attached.  On the opposite of the hole, at right angles to the rubber flange, are attached two rubber lugs.  The rubber flange is placed between the lips, and the lugs are held by the teeth.  The rubber tube of the mouthpiece is connected to the tube carrying the valves.  The nose must be tightly closed if mouth breathing is used.  This is accomplished by a nose clip, which consists of a V-shaped metal spring, the ends of which are provided with felt pads.  A toothed ratchet is attached to the ends of the spring, and serves to hold the spring tightly clamped on the nostrils in the proper position.”

“Some individuals experience great distress when made to breathe through the mouth.  For these it is best to use a face mask.  Unfortunately at the present time no mask is entirely satisfactory.  Perhaps the best is sold by Siebe, Gorman & Co. which is pictured in the cut.  After being placed in position the face mask should be tested for leaks, which can be done by putting soap around the edges.”