No headstamps can indicate very early cases, as not all manufacturers bothered with them at first. This is not a guarantee though.
It appears from the pictures that some of the cases look quite a bit nastier than the others. If all of them received the same amount of cleaning and were found in the same general area, it's safe to assume that the additional degradation would suggest a longer stay in the ground and thus, probably a significantly older age. A lot of corrosion (particularly at the case neck and inside the case itself) can also indicate the use of black powder and/or the harsher priming compounds originally used, but again, not always.