Here in Spain I've always seen the backglass I've used for the UVP. Probably this machine was also sold in Germany (manual in IPDB is in german, and I downloaded some time ago a couple of pictures of the cards in german), so maybe this alternate version (very similar to this one) was the german version?
Before every ball, it sounds a Lili Marleen cut, but you can choose to hear "The Bridge on the River Kwai" theme. I'll try to explain in English how to do it 
"End" key toggles the Interlock-Switch. "7" key is the "Test" switch. You should:
1. Press "End" to "open the door" and release the interlock-switch.
2. Press "Start" (usually "1" Key), and you'll see "boop 01" on the displays.
3. Keep on pressing "End" key until you see "adjust 20" on the displays (you need to press twice "End" key to keep on advancing, since every press toggles - not presses - the switch).
4. When you see "adjust 20" on the displays, you'll also see "0" on the left bottom display. "0" is Lili Marleen, "1" is River Kwai. Now press "End" key once (apparently nothing changes, but you're now "releasing the Interlock-Switch"). Now press "7" key (Test Key) and you're toggling between "0" to "1" on the bottom display. Select the melody you want to hear.
5. Finally press once "End" key (to press "interlock switch" - close the door) and press "Start" key to return to game mode.
I now see that I shoud have used two keys to manage interlock-swtich on the script: one for toggling and other for pressing+releasing. This way things would be easier.
Regards.
BTW, Lilli Marleen is better by far 
Edited by MRCMRC, 12 October 2016 - 06:46 PM.