Ok,
I've spoken with the shop assistant who sold me the bass, with the owner of the shop and with the former owner of the bass.
Now I'll try to explain it.
The shop assistant (which is quite young and hasn't worked there for a long time) didn't check the serial number well. He saw that the bass was an original MM..and didn't give any importance to the hole in the bridge..which is his mistake.
When I told him about the possibility the bass was stolen..he was really surprised. I am sure he was sincere, because his astonishment was really genuine.
I saw his face and I believe in "musician goodness".
We spoke about it with the owner of the shop, and we called the former owner of the bass. He told us that the bass, that he owned for long time, was a present. His uncle (or his cousin..don't remember) bought it on internet and gave him as a present. He told us he never changed the bridge. I spoke directly with him..and in my opinion he was sincere.
To sum up: the shop did a mistake, but in "good faith" (Don't know if it is the right word, sorry). They allowed me to speak with the former owner..wich seemed to be in "good faith" too.
(Could someone sell a bass in a respectable shop, knowing it had been stolen?? And could a shop that has "something to hide" allow me to speak with the former owner??)
Now I am thinking about what to do...
I can't know for sure if the bass has been stolen...or if only the bridge comes from a stolen bass. I can't know much about his history, so I can't be sure about anything (as Bovinehost said)
If I give the bass back to the shop, what they (probably) will do would be this, I suppose:
They will substitute the bridge with a new one, and will sell the bass like that. Then the people who will buy it will not know anything about this history, and the "bad kharma", as someone said, will go on and on. It will be increased, according to me.
So, if I give the bass back..I won't solve the problem.
If I keep the bass (wich I truly love), I will be the owner of a bass...wich came to me in a strange way indeed. But I will not sell it anymore. And the bad kharma will stop. I will love it even more than his first, unknown, owner.
What do you think about that?