My notebook is an E5420 and a quick search gave the T54FJ as the compatible battery type. It seems to be quite common (also used in 5520/30 and 6420/30), and it was listed for these models.
So what do you think now: is it an old Dell, or a not-so-old not-so-Dell?
This model pre-dates your battery date code, so it is very possible the battery is five years old. If that's a genuine Dell battery, it was indeed built in August, 2012.
Right, so I meant the battery model of the original one for your system is listed as 5G67C (based on the serial number you provided in the Private Message you sent me, and you should see this after the first few digits), this number does not reference the model of the system or all the compatible battery types that work etc., just the actual battery model that it is (internally etc). The reason I think the serial number on the purchased battery is off/not real, is because there is not a clear part number included with it like there should be, and it has been like this for a long time (for this scheme). This even works on other Dell devices like monitors etc.
The battery you bought does not contain the model number within the serial number* and has 1 to many digits, so either it is very old, or generic like I mentioned. I am thinking it is generic just because it doesn’t contain the internal model number within it.
Let me know if you have any further questions. And ejn63 is correct on the time if it is real, but I do not think it is.