You need to remove the top of the casing and looking inside you see, that the connector is threaded and a ring with slots on both sides tightens it on place. Loosen that ring and you can remove the connector. You need to desolder the wires of course. Sounds easy, but it ain`t. The copper wires are frail and snap most likely.
The bass roll-off coils are made of particularly soft copper and tend to break and then you need to replace the coils. Then just find a fitting XLR with a proper thread.
The plastic frame is just one of those lousy "improvements" done to save expenses I guess. It just tends to brake and no glueing would hold forever. A thick layer of epoxy is likely to hold, but how does it look like. I have had best results soldering with similar plastic and sinking in three metal pins to strengthen the joint. Afterwards I try to make it look as original as I can. Not easy though, but with a lot of practice....
Lots of stories around about the D12, D20 capsules. I have never found any proof of any significant difference in the quality. In the fifties mostly(always some exceptions)D12 and D20 used the same capsules, but then coming to the sixties the D20 and D25 had the whitish "paper"-diaphragm, whereas D12 had a transparent diaphragm and that in many variations too.
Always variations too many to even start with, but the basic sound-properties remained within tight enough limits. One important detail is, that D20 and D25 always had 60-ohms capsule. D12 had either 60 or 200 ohms. To alter the output-impedance a transformer was used or not.
Did not mean to fill the thread, so I leave it at that.
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