Hi, Mainsail;
The model 56 is an S&W model number, and it is the precursor of the model 15-2, and was developed for the USAF. 15,205 manufactured between 1962-63. Most are believed destroyed. Four ANIB specimens are at the Springfield Armoury Museum. "Might bring in the $2500.00-7500.00 range. A couple of sales in Excellent condition have been reported around $4000.00-5000.00."
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The above is from the Standard Catalogue of Smith & Wesson by Supica and Nahas (third edition). I can highly recommend this book to anyone interested in S&W's.