A warm welcome to the TMF ! What a wonderful way to start a Sunday with the “discovery” of a really splendid Xb EMG is such perfect condition---very many thanks for posting details.
I am sure, by now, you will have seen the many discussions on this forum in relation to EMG/Expert gramophones. Your example has particularly wonderful and appealing finishing paperwork in addition to a fine case---a real credit to you and former owners. As you say, a surprising survivor through the vicissitudes of life. Superb and well done.
I think it may well be pretty rare and I have only seen one other which was sold at Bonhams a few years ago. Its hyper-rare (unique ?) bigger brother is owned by Ian Maxted and can be seen here:
https://www.youtube.com/watch?v=J80jNyNS004
This video shows the Oversize Export horn which, with a final bell mouth diameter of 33 ½ inches, is 4 inches larger than your own. The effect of this is to increase bass response from say 100Hz to approximately 75 Hz. There are thought to be only about a dozen original Oversize instruments left on the planet apart from a few examples with new horns which incorporate other original parts.
As far as dating is concerned, the whole area is rather uncertain but you may find the following link will throw a little light on the subject:
http://forum.talkingmachine.info/viewto ... 11&t=15383
Your case has the lateral strengthening gussets on the lid to arrest incipient outward-bending inclinations experienced on earlier cases which had pegs to try to achieve a similar result. You also have the later single side hinges, as opposed to the earlier single full width piano hinge.
Your questions regarding the soundbox are interesting and it is certainly extremely unusual to find the use of a mica diaphragm in such a late soundbox. Similarly, in general terms, a circular needle holder was long since superceded by the early nineteen-thirties in my experience---the majority had triangular holes enabling the use of all types of needles---steel, bamboo or thorn. Maybe you have a hybrid (factory or subsequent enthusiast ?) for the reasons you suggest and it would be fascinating to compare the sound generated by yours and an aluminium equivalent. Perhaps you have already done this ?
Your musings as to provenance prompt a story of my own Expert Senior which was found by the previous owner over 50 years ago in a Wigan (Lancashire) junk shop, bought for £5 and transported home in the back of an open two-seater sports car !
In any event, you are the owner of one of the most highly regarded acoustic gramophones ever constructed and have clearly demonstrated you are absolutely the right man to possess such a thing.
Many congratulations !