I recently picked up a Model 51 that is from the second year of production, very early serial number and very nice condition. This is one beautiful pistol, both from a cosmetic point of view and from a design point of view. I love it and hope that yours is as functionally nice as mine is. Mine is early enough that it has a HORRIBLE trigger pull but that was apparently designed out in the models produced later. This is a well known issue with the earliest serial numbers.
The only bad thing is that finding replacement springs and such can be a problem. Wolff gun springs makes springs for lots of pistols and rifles but the Model 51 is not one of them.
Enjoy your pistol.