A few years ago, I purchased a WW2 German Feldfernsprecher 33 (FF33) field telephone that had a handset with a rubber insulated cord. I was told that after the war, the Deutsch Bahn (German state railway) used some FF33 field telephones on their private telephone system. These phones received rubber insulated cords, so, I would assume that this handset came from one of these phones. I decided to convert the handset cord back to a cloth insulated cord.
I acquired the new cord from a company called PhoneCo, Inc. They sell restoration parts for antique telephones. The new cord has 4-conductors and is available on: https://www.phonecoinc.com/category.asp?rpage=search&category=Cords#02005
Incidentally, there are five pins on the handset plug, but the center pin is not connected to any wires. Pins 2 and 4 are for the handset speaker. Pins 1 and 5 are for the handset microphone.