Pennsylvania Tank Line first appears in the February 1912 ORER with a picture of PTX 1001:
https://play.google.com/books/reader?printsec=frontcover&;output=reader&id=QRs6AQAAMAAJ&pg=GBS.RA1-PA522

Their entry used a picture of PTX 1061 by December 1912:
https://play.google.com/books/reader?printsec=frontcover&;output=reader&id=tboMAQAAIAAJ&pg=GBS.PA998

In May 1915, the picture changed to PTX 1091. Decent picture here:
https://play.google.com/books/reader?printsec=frontcover&;output=reader&id=ZPg4AQAAMAAJ&pg=GBS.RA1-PA849

The change to the "new" underframe appeared on PTX 3500 in May 1917:
https://play.google.com/books/reader?printsec=frontcover&;output=reader&id=O_c4AQAAMAAJ&pg=GBS.RA1-PA915

The more familiar cast tank saddles seem to have started circa 1918.


Standard Car Equipment Co. first appears in the January 1916 ORER. The first picture appears in February 1917:
https://play.google.com/books/reader?printsec=frontcover&;output=reader&id=ZPg4AQAAMAAJ&pg=GBS.RA1-PA945


And as a bonus, here are some pre-1917 German/General American tanks:
https://play.google.com/books/reader?printsec=frontcover&;output=reader&id=Wgs5AQAAMAAJ&pg=GBS.RA1-PA923
https://play.google.com/books/reader?printsec=frontcover&;output=reader&id=Wgs5AQAAMAAJ&pg=GBS.RA1-PA943

David Thompson