[m0cemdave]

They are Acos inserts, but I've never been able to find out the part number. But the well-known Acos 43 is 43mm diameter and these are 1" (25mm) which may give a pointer. There was also a type 38.

Mine have the green markings, and they measure around 1.5mV RMS with speech at 6".
I used one, mounted in a block of soft foam, to replace an old 45mm Rion that had suffered from moisture ingress. After a few years it failed (sounded muffled and low level). I don't know if it was the dreaded moisture or if it had been dropped, but I swapped it for another which is still working fine. So it may be worth checking yours by cross-comparison to see if they all sound similar.

The thread is 6BA.

Note that the terminals will not take solder - I wired them up using inserts taken from a small multipin socket.