Here's a theory about why so many S&W's were shipped in March, 1946:
During WWII the Office of Price Administration (OPA) set the upper price for everything from wheat to steel, including consumer goods. After the War the price controls gradually came off one by one for a period of about a year and a half or so.
In a search on the subject I was able to find specific dates for the elimination of price controls for various things, but so far not for firearms. So, this is slightly speculative, but I think that the common March, 1946 shipping date for numerous S&W's is because that was when the price controls for guns were removed. (More $ for S&W)
Consider this a good theory until someone can find the specific date for the removal of OPA constraints on the firearms industry.