I haven't found enough of them that were legible to draw a conclusion, We also don't know if the provider did batches ( a bunch of Mustang ones for a couple days then a bunch of Fairlanes...) to use the possibly limited stamping machines more effectively like the muffler supplier. If so there would only be a couple or a single dated for most cars again like the mufflers.
I'ld have to look at my pictures (away at the moment) to get an idea of coding and of course would need to know what year your working on since there were at least a couple suppliers of the base metal sheets the tanks were made from
Again - sorry not enough info right now for me.