Antonio, I'm not at home right now but have my 8 month old H3D/31 with me and the serial number is DC57055XXX. Does your serial number begin with DC ?
I recently upgraded a H3D/39 which was a factory upgrade from an older H2D/22 to a H3D/39 which retained it's earlier serial number. IF your camera is an upgrade, it'll tell you briefly on the LCD when you first start up the camera ... otherwise it's irrelevant since H3D/39s are a recent model and can't be that old.
The only other concern would be if it was a demonstration model sold as a factory fresh model. IF it was a demo, the seller should have told you that it was.
Again, I urge anyone buying one of these expensive digital backs to research the dealer completely ... personal and knowledgeable after-sale service is extraordinarily important.
In my experience the big box stores (large outlets like B&H and Calumet) aren't as good at that kind of after-sales service than some of the more dedicated MF digital back dealers. Sometimes the big box stores will dump old stock at an apparent savings, but can't compete when it comes to the kind of service that such expensive products deserve.