Thanks Toril.
Indeed I really enjoy to exploring every little piece of a new instrument like this.
The feeling of new instrument can't compared with anything. Imagine how I was feeling until this had to arrived during its far trip from Germany to Greece, that it took almost 20 days! But fortunately everything went fine with the transportation, so now I can enjoy this little gem.
About the battery case, it seems that this model maybe was designated to work with batteries too, like the psr-e series, but maybe they changed their mind last minute. But as I can see, it says 6x1.5volt batteries, so this voltage would not be enough to power this instrument.
The most weird is the 1990-1994 year date stamp on this case, but who knows, maybe they did this on purpose, to give at this instrument a 90's touch at its spirit.