Here is a copy of the list I made from the above posts, for my own sorting and use.
Pertinant comments are listed in: -?[...]?

The following is a list of the manufacturer’s names that are found on the sign in this book. Most of the major companies will be present as they produced high quantities of signs. Street addresses are not listed because some companies did business at more than one location. Many companies listed were manufacturers of very different products such as stove castings building panel’s kitchen ware & other utilitarian items. Although it is difficult to establish the dates listed will give you an idea as to a particular company's heyday. A (*) designates that limited information is available for the years of production. It is of interest to note that one of the most frequently encountered names found on signs EM Burdick of Chicago, was a distributor not a manufacturer. This is true of other companies as well.

Action Burrows Co Toronto Canada 1900-1920
American Sign Company 1920-1930
American Valve & Enameling Crop. 1928-1970
Baltimore Enamel Baltimore Maryland 1897-1943
Beach Coshocton Ohio 1910-1940
Beatty McMillan Co Detroit Michigan (*)
Billy Newton Company Minnesota 1930-1940
Brilliant Manufacturing Co Philadelphia Pennsylvania 1910-1930
Burdick Chicago Illinois 1895-1950
C Robert Dold Offenburg Germany 1905-1920 50s-60s?
Cameo Gilla 1904-1980
Chicago Vitreous Enamel Product Company 1930-1980
Crichton Curl Enamel Co Ellwood City Pennsylvania 1905-1920
Enameled Iron Co Beaver Falls Pennsylvania 1892-1900
Enameled Steel Co Chicago Illinois 1900-1920
E Francis & Sons LTD London SE
FE Marsland NY Hartman Steel Co Ellwood City Pennsylvania 1890-1910
General Steel Wares (GSW), Toronto, Ontario, established 1927
Imperial Enamel Co LTD New York 1890-1910
Industrial Signs Incorporated 1940-1950
Ingram-Richardson Beaver Falls Pennsylvania 1901-1960
LD Nelke Signs New York 1900-1935
Maryland Enamel & Sign Co Baltimore Maryland 1900-1920
MC Math Axilrod Dallas Texas 1930-1965
Mulholland Philadelphia Pennsylvania 1925-1940
National Enameling Co Cincinnati Ohio 1895-1915
?Nesco Inc Signs New York & Toledo Ohio (*)
Orme Evens & Co LTD New York 1895-1915
P&M Orilla Ontario Canada 1930-1950
Pennsylvania Enamel New Castle Pennsylvania (*)
Porcelain Enamel Manufacturing Company 1910-1950
Porcelain Medals Incorporated 1925-1950
Reliance Advertising Co New York 1915-1930
Robert Deto Offenburg Germany
Sales Service Corporation Chicago Illinois 1920-1930
Shank Sign Co New York 1920-1930
St. Thomas Metal Signs, St. Thomas, Ontario
-?[Never seen a porcelain sign marked S.T.M.S. (St. Thomas Metal Signs, St. Thomas, Ontario) only silk screened tin signs. Dave GILL, Dave's Garage & Memorabilia, Inc]?
-?[I believe St. Thomas Metal Signs made the porcelain Coca Cola thermometers with the silhouette girl at the bottom. They are marked ST. They probably though were mainly a tin sign maker. I have a few Canadian porcelain sign that are GSW. Cheers, Don “COOPERSPLACE” Edmonton AB. Canada]?
SWP South West Porcelain Sign Co. Tulsa, OK
Tennessee Enamel Manufacturing Co Nashville Tennessee 1925-1970
Texas Neon Advertising Company 1940-1950
Texlite Dallas Texas 1920-1960
Union Enamel Co LTD Burbank
Veribrite Signs Chicago Illinois 1915-1965
Vitrified Iron Products Co. Clyde, Ohio
-?[seems they made porcelain sign up into 1933 and then closed up the doors and were reopened in 1934 making porcelain refrigerator panels]?
-?[My guess would be that NESCO a misspelling, not made by you Gasoildude - DOC didn't find the Wesco entry in Rex' list either.]?
WF Vilas Co LTD Cowansville FC 1925-1950
Walker & Company Detroit Michigan 1925-1960
Wolverine Porcelain Detroit Michigan 1925-1945

!!! Remember anything made buy AAA Sign Co is a REPRODUCTION!!!
!!! DESPERATE SIGN CO. are 90s DATED Fantasy Signs that usually have the date concealed in some way!!!

Dave(509)847-5893 LOOKING-4:WELDING SIGNS,Charter/Finance/PulOil/Trulite/TruTest-Oil,FREEDOM cans,True's gas CORALINE/RAINBOW,FLYING A,Wa/Or/Id Chief!