[Arran]

Who knows when the attachment was added? The 620 was a 1936 model so it would have been five years old when the U.S entered the war, it could have been added before that time, at that time, or at any time after. Maybe it was used in a hospital or maybe someone added that feature for home use.
There was another aftermarket accessory that was for sale in those days called a "Radiolink", it was a cheap way to pipe in radio programs from one room to the next through a jack installed on the back of the cabinet and through one or more external speakers. I have a set that had the jack still attached to the back of the cabinet and the junk shop that I got it from had one of the speakers that I guess they thought was some type of collectable and was there until the day the shop went out of business. It was just a P.M dynamic in a plain leatherette covered wooden case, it probably never sold because they had $30 on the thing. Not really a collectable much more of a curiosity really.
Regards
Arran