Just measured my DJ-500T, it's 10mm center to center. What radio do you have?
It's 10mm. I derped and haven't used my Executive Pocket Chum in many years; no other micrometers left in my tools. A year or so in these bifocals and I still haven't adjusted; it didn't help matters much. I had to calibrate on paper with a precision ruler and found my error using the Pocket Chum. Sorry about that.
Also, I didn't see any of the messages in between mine. I assume you are on moderate because you are new. That's some very good information you posted and I appreciate it. Other DJ-500 2nd generation owners will find this stuff useful for years to come, as we know, information on this radio is light on the web.
So, it looks like the middle part on the 2.5 mm jack is NC as expected. That's good. Like I posted, there are only two rings on the each post for this Motorola 50225 and it works so I have to suspect that there really isn't 5V on that ring from the DJ-500???
My lousy measurement shows 14mm length on the 3.5mm diameter post and 11mm length on the 2.5mm diameter post for the Motorola 50225. It seems to fit in easily and has never come out by accident, even when losing control of the body of the radio and holding the mic. Please, take my measurements with a grain of skepticism. I had to take off my bifocals and use a magnifying glass.
EDIT: I am butchering: tip, ring, and sleeve all over these posts. There is only tip and sleeve for each post on the Motorola 50225.