High Imperial Roman army vs Late Roman army

[Flavivs Aetivs]

Actually he writes that the pilum is now called the speculum, and that it was a throwing spear (unlike which you wrote earlier, that it was used for thrusting).
They had likewise two other javelins, the largest of which was composed of a staff five feet and a half long and a triangular head of iron nine inches long. This was formerly called the pilum, but now it is known by the name of spiculum. The soldiers were particularly exercised in the use of this weapon, because when thrown with force and skill it often penetrated the shields of the foot and the cuirasses of the horse. The other javelin was of smaller size; its triangular point was only five inches long and the staff three feet and one half. It was anciently called verriculum but now verutum.
So in this passage, he sees no difference between the former pilum and the current speculum, whereas you say that this speculum looked different from the pilum (with a leafshaped blade). I’m just saying: either the pilum remained in use under a different name, as Vegetius suggests, or Vegetius was wrong and the pilum was replaced by a new weapon, the speculum, which looked differently. I’m going for option 2.

I think Ammianus may be fond of using older words, but that we agree on this, it need not be wrong, like you I don’t think that Ammianus had older weapons in mind. I think that the terms he uses are more generic. I don’t know what that tells us about plumbatae, but so far the finds of these from Syria and other eastern provinces stand out through their absence..

Robert's right on that, it was like a pilum with a leaf-bladed head, so it could double as a thrusting spear if need-be.

But I'd also ask that when ammianus says "Gladius" could he be referring to the Semi-spatha? Of course he could be calling a normal spatha one, but still.