A set of Trimm headphones manufactured by Trimm Radio MFG Co. Chicago USA.

A set of Trimm Dependable headphones manufactured by Trimm Inc. Libertyville Illinois USA.

Another set of Trimm Dependable headphones manufactured by Trimm Inc. Libertyville Illinois USA.  

A set of headphones manufactured by the Federal Telephone and Telegraph Company of Buffalo NY USA. The model number for these headphones is Type 53-W and they are 2200 Ohms.

The photograph on the left is a set of headphones manufactured by Telex and the model number of these is 6101. I believe these have an impedance of 6000 Ohms. The picture on the right is a headphone and microphone combination manufactured by Western Electric Bell Systems and having a model number of 52.