The dates are the equivalent of a "form number" and date on something like a government form. In most cases they indicate when the sign layout was last changed/last stencils made as opposed to an actual manufacturing date. When Texaco called their sign manufacturers and ordered a quantity of Fire Chief pump plates, the order would include something along the lines of "Revision 3-47" to indicate first off that there was no new version unknown to the sign manufacturer and as a reference not to use an older design/stencil in error. Most revisable printed or screened products use a similar system - many oil cans are dated, too, some with open dating, some with a code, indicating which form design was used.
