Shure Green Bullet
There are numerous versions of this since the late 40s produced microphone from America.
The many terms such as "CR", "CM", "black label", "white label" ... that you here, do not interest us particularly. Anyone who wants to get rid of his money here, must know what he is doing, because these days there are a lot of different models with very different properties. However, the many years and the usual manufacturing tolerances can certainly bring a "baby" to the patient collector which has a particular sound. Who wants to read about the history of this classic, take a look here:
In the 1970s, the production has been moved to Mexico shortly after the introduction of the 520d and thus acquired new equipment. Therefore, the tolerances of the mexican elements are not as big anymore, and they are all "Dual Impedance", so you can not take the wrong "type". If such an element does not sound, then it is broken (usually by a user error, because it is robust) or you have had very bad luck.
The picture shows an element from a 520d from 1993, already advertised as harmonica microphone.
But even with this microphone, there is a drawback. The 6m cable is securely connected and regularly annoying. A practical XLR connector does not fit to the case. A coupling on the cable is somehow unprofessional. Only one solution remains, the good old Switchcraft Connector. For a microphone the acquisition of this tool is not profitable.
If you still want to add a potentiometer, it gets hard. A drilling template is advisable. Then a 16mm alpha or a 9mm mini potentiometer fits.
The 520DX exists since 1996. Instead of a CM capsule, a huge dynamic capsule with a transformer can now be found - means "High Impedance". It has a volume control and is produced for harp. Even if you read sometimes, it would be a bad microphone, no, it is still the standard. A little cleaner in tone than the 520d but it may also be more universal. Anyone looking for a starting point where nothing can go wrong, take this. Not much choices in harp microphones off the rack.
The picture shows a model of 2014 - now with thinner cords and CE mark.