All Beovision 4's have a grey B&O label on the back that gives the serial number and the type number. It will also give a production date. From this information you should be able to determine the D#. Sometimes you need to reference the serial number. Example all D-X panels start after serial number 1XXXXXXXX. If you connect any panel to a Beosystem 3 the service menu of the BS3 will show you all the panel info on screen. Ex Monitor 1 = D10 serial Number 1xxxxxxxx, type number XXXX. The production date with my previous info should get you close.
There is also the exterior design.
1. Earliest panels where bronze colored and just had a Bang & Olufsen logo painted on where the panasonic name would have been. D4, D5 or D6?
2. Next panels included a brushed aluminum square edged frame in Black, silver or dark grey. Later red and blue added. D7, D8, few D9.
3. Then bump for remote control eye was added to top right corner and the frame has a curved shape. D9, D10, D11