I think I've seen that one before on Armslist and it was flagged then.
However, the Model 56 had it's own serial # range and were only produced in '62 and '63 before the Model 15 2 inch began in 1964. Reading some of the posts it appears that there are only about 26 known Model 56's that are verifiable. If you Google "Model 56" you will find a lot of good information on this intriguing gun, but here are a few for starters...

Help ID'ing a model 56

Final Update: I DID buy this rare Model 56