Followup:

I was going through some of my strikeold junk/strike valuable
antiques and I found a couple of the Realistic/Archer/Radio Shack car
stereo plugs. They have five pins, and a keying notch on the outside of
the plastic body. If you hold the keying notch at 12 o'clock, the five
pins are arranged like this (use a fixed-width font):

* *
*
* *

Some measurements I made:

Diameter of plastic body of plug 0.614" 15.6 mm
Diameter of each individual pin 0.093" 2.4 mm
Length of each individual pin 0.375" 9.5 mm
Distance from edge of plastic body to first row 0.213" 5.4 mm
Center-to-center spacing of pins on first row 0.295" 7.5 mm
Distance from edge of plastic body to third row 0.413" 10.5 mm
Center-to-center spacing of pins on third row 0.327" 8.3 mm
Second row (one pin) appears to be in the center of the plug

There is no locking mechanism; it just depends on the fit of the pins.

Matt Roberds