What is the part number for speaker-elements of these headphones?

The left side of the Revox 3100 headphones was mute, and I decided to check it out. I measured everything with multimeter, and the right speaker-element was fully working, and the left was broken. As you can see from the picture, there are no pert number on the speaker-element, not even on the other side (I had removed the cloth, too).

Does anybody know the part number for Revox 3100 headphones’ speaker-element, or at least for a fully compatible part? All I can tell is that these headphones are 600 Ohm headphones and that the diameter of the speaker-element is 41 mm.

Thanks.