compression driver markings

[trekman]

What are the numbers frequently written in magic marker on the top plate, which is what I am calling the silver ring typically visible around the driver under the cover? Typical values would be 0.2 or .4 These suggest some sort of asymmetry to me. thanks.

[Steve Schell]

I believe those marks indicate the height of the phasing plug in relation to the top plate. When drivers are assembled, tolerances can stack up in various ways. Shims are sometimes used under the diaphragm mounting rings to obtain the correct spacing between phasing plug and diaphragm, a critical tolerance.

Those crude El Marko graffiti scribblings look so out of place on a finely crafted JBL transducer, but they've been doing it that way for decades.