grundig: 3397 Stereo; Konzertgerät

Hello !
I've got this model for a few days, and I've some questions :

1) On the front face, at the right of the "Aus" button, there is "something" blue on the RM picture. This is not on my set. What it is and what is the purpose ?

2) My model is written "3397 Stereo" but the schematic of the RadioMuseum is labeled  3397/3397a. My set has an EBF89 tube and the schematics spoke about "EBF89 o/EAF801. Is it a functioning difference , May I switch from one tube to the other or does the wiring require some modification ?

3) I do not have the stereo decoder for this set. I plan to install a cheap IC based decoder while I wait for the proper one to show up.  Should I cut the dotted wires showing on the schematic  into the decoder plug between pins 2,3 and 5 ?  How suppress the mono de-accentuation ?  Is it  automatically made when switching from mono to stereo ?

4) What is the best way to insert a stereo indicator ? (the IC decoder have a LED output for this but another solution is feasible).  Is it possible to use the EM87 for this purpose ? What about replacing it with an EMM803 ?

5) I do not fully understand the wiring from the decoder plug. I need a switch for the mono/stereo decoder. Is it available in the plug ? If not, is  it feasible to pick a switch from the keyboard ? (I'm far from familiar with the type of switches used into this set and with the drawing Grundig uses, so I do not understand very well the wiring.)

Sorry for asking so many questions, but I've not made any electronic in the past 30 years and I was far from genius back then.....

Thanks a lot in advance for your help and advices.

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