The sound box that came with it is NOT the original one. Here's what it should look like --


http://www.phonoland.com/viewtopic.php?f=7&t=557
Also, here's one I culled from a screen capture of a video, which would match the plating on your tone arm --
The one you have probably came from a circa 1940s portable. If it's restorable, you could probably use it until you find the correct one. (That too, would probably have to be overhauled.) Then you'd have a nice spare, at least. Sound-wise, it would probably perform as well as the original one, once it's repaired.
In answer to your question about the diaphragm, the Tru-Phonic sound box was also fitted with an aluminum diaphragm. Most of the sound boxes manufactured after the switch-over to electric recording in 1925, in fact, were. Aluminum is more flexible than mica, and does a better job responding to the more demanding groove cut of electrically recorded discs.
It also looks like someone replaced the original grille with a more modern, plainer version. Normally, the grille would be rather fancy with cross-supports or some type of fretwork.
OF