To determine the month and year of manufacture on most Dietz lanterns made between 1915 and 1956, look at the "M" or "S" production date located under the patent dates, usually located on the upper part of the air tube to the right of the fuel cap, or on the center air tube on Hot Blast lanterns.  (Do not confuse the "M" or "S" production dates with a patent date when looking at a Dietz Lantern.) "M" denotes Dietz New York City "Main" Factory #1, while "S" denotes Dietz Syracuse Factory #2, which is followed by the month and year of production.  Stamping Patent and Production dates into Dietz lanterns was abandoned in 1956, coinciding with the establishment of the Hong Kong factory.  (i.e. "S-6-41" stands for Syracuse Factory #2, June of 1941, "M-1-25" stands for New York City Factory #1, January 1925.)  Note:  Lantern production ceased in the New York City "M" Factory #1 in 1931.