In researching a 1919 Cream Pitcher I have which was made by Homer Laughlin, I came across the following information from scanning a book called "Homer Laughlin:  Decades of Dinnerware."   This is a paraphrase of some of that information.

These pieces have very confusing backstamps, because they were changed so often, and some patterns have no backstamps. 

HLC was originally located in East Liverpool, near the Ohio River, but that plant was gradually closed by 1929 and all production afterward was done at the plants in Newell, W. Va.

The backstamp system used by HLC differed depending upon the decade of production. The company did not include dates on its dinnerware before 1910. 

From 1910 to 1919 a three-character code -- two numbers and a letter -- were used.  The first number indicates the month it was produced.  The second number indicates the year, and the letter N was the third indicator, which means it was produced in Newell, W. Va.  A code of 2 5 N would mean February, 1915, Plant N.

From 1920 to 1921, the code was changed.  The year was indicated by 20 or 21.  A code of 6 21 N would mean June, 1921, Plant N.

From 1922 to 1929 a letter was used instead of a number for the month of production - A for January, B for February, etc, with one exception -- the letter "I" was not used.  A single number was used for the year.  So A8N would mean January, 1928, Plant N.

Beginning in 1930, they kept the letter to designate the month, but began using two numbers for the year.  The plant designation stayed the same, although some of the plants have both a letter and number designation -- the number being the plant building number.  So D38N5 would mean May, 1938, Plant N-5.

Many patterns produced after 1930 have no date or production information in the backstamp, but other patterns retained the system up to 1959.  It appears the system may have been discontinued then.