First the ones I have pictures of and have seen are not stenciled on the part but instead appear to be stamped
The code appears to be a manufacturers date code with a couple of additional codes to identify plant and possibly shift, though all of the ones I have pictures so far show onl "PB" as the extension/end of the code. Often has addition hand marking as well as the decals on the upper wheel house extension
Rear tank/vent is also normally stamped and the forward tank/vent is not always stamped int he location shown above