Hello Kolja, and welcome to the forum!
Even though it was damaged in the storage you described, you have what looks like a Luger in it's original finish.
Luger collectors value pistols that are as close to original factory condition as possible.
That means that the less that is done to a Luger, the better!
The original grips on your pistol are particularly valuable. This type of Luger has been called a "black widow" because of the black bakelite grips. This is a marketing term started many years ago to make these more attractive to buyers. It worked, and a demand developed for this variation of Luger.
Your byf 42 Luger s/n 9416h was made toward the end of Luger production by Mauser, just before they moved over to P.38 pistols.
It is normal for some red oxide to develop over many years of storage. The areas that are pitted happened because this was allowed to develop too long without applying preserving gun oil and cleaning.
It's possible to remove this red oxide rust by using an oiled #0000 steel wool or bronze wool gently. If you don't have experience doing this (you must use good judgement and not rub too much) just keep the pistol properly clean and oiled.
Have you disassembled your pistol to see if it is all matching (the numbers will all be "16" or "9416" on the pistol?
Be very careful if you remove the grips. The left grip should be just barely lifted off it's mount. These break easily near the safety lever base if they are lifted too far before sliding them off the frame.
My last thought is that you should not try and fire the pistol. It will probably work fine, but if you break a numbered part, the historic and financial value will be reduced. If all matching, it is probably worth between $1300 and $1600 to a collector in the USA. You can see retail pricing at websites like
simpsonltd.com . Those prices would reflect insurance replacement value.
Keep it clean and oiled. Store it in a dry place. Use the silicone impregnated woven gun socks to protect it from moisture.
You have a fine piece of history - the history of your family and your country!
Congratulations...
Marc