Western Electric strange mouth piece

[tom128]

While I was going through a parts box looking for something else I came across this mouth piece that I had forgotten about. It came on a WE 151 stick phone that I bought four or five years ago from an Iowa seller. The phone was really dirty, splattered with paint, bad finish, bad cords, bad felt, etc. and in need of a total restoration. This was the only telephone item the seller had listed on ebay at the time. If I remember correctly it was listed as an AT&T model 337 with no reference to Western Electric or 151. It had one up close photo of the transmitter face plate info and otherwise fairly grainy pictures. When I got the phone I discovered it had the grove in the mouth piece. The phone has a 337 transmitter, stamped out 51AL on the back of the perch, restamped 151 on the front of the perch, 706A receiver, and a #2 dial converted to a #4 with a 1939 Metro face plate. Inside the phone everything looked correct and hadn't been touched in years. It was marked with a rebuild stamp of 5-33.

I'm just curious if anybody has seen anything like this in the past. The groove doesn't appear to be home-made, it is too accurate all the way around. Much like a 706 receiver and cap or a later E1 handset.