I spoke mainly about the blade type when I stated the ubiquity of the design. My source, the RGA is the german standard lexicon on migration period european archeology, Marcin Biborski is an expert on his matter and I dare to trust him on this. This is also consistent with actual archeologic evidence. .
Behmer wrote his book "Das zweischneidige Schwert der germanischen Völkerwanderungszeit" in 1939. This makes it difficult as a standalone source twice, as on the one hand, it does not incorporate any find or analysis made in the last 7o years and on the other hand was written during a period which is quite problematic for "germanic" archeology when people like Kossina gave a direction to the matter which tended to mingle with the nationalistic ideas of the Nazis.
The regionalistation of types based on the location they were found is also problematic.
In the context of the bog depositions a carriers of these items travelled a long way to meet their fate, also, weapons were trade goods and prestigious objects and so highly mobile.
A better and more recent source is the Dissertation of Christian Miks "Studien zur römischen Schwertbewaffnung" (Rahden 2007).
The Hilt design which you identify with the Behmer Type 1, true enough was only found in the north. On the other hand, how many complete 4th century organic hilts do you know of beside these?
They do survive only rarely. They are organic, which means that they tend to decay fast in most kinds of soil.
So, when we look at the sword tangs, which are quite short on late 4th and early 5th century spathae, we can deduce a short hilt and slim guard and pommelpiece. Fifty years later, we find the first gold hilt spathae, like at Pouan above or Tournai, which also have the same kind of guards.
When we look at the middlepiece of the hilt, we can see a clear connection to the classical roman hiltstyle, as both share fingerlobes to give a better grip.
So we can set this design in between the classical roman hilts and the early migration period "germanic" hilts.
The difference are, on first glance, the rounded guards of the roman sword hilts and the straight flat guards of the "germanic" spathae.
But what's "germanic", anyway? The bulk of the bog swords can be classified as roman and the rest are roman blades with homemade hilts.
As you mention Illerup Ĺdal, which is of course a bit older then the Nydam horizon where the sword we talk about was found, even the bulk of the swordbelts and baldric were of roman manufacture.
I don*t think that the differences between swords in Germania Libera and the provinces are so big. The people which used them in this timeframe were the same on eac side of the limes, anyway.
The roman guards tend to flatten out in the 3rd century anyway.
So by a stylistic analysis we can state that the hilt desgin we are talking about doesn't have any significant features which determine a babaric-germanic origin.
Another point is the fact that on the other Nydam hilt of the same style (yes, actually there are two hilts of this type in Nydam, one of which was lost in the last century, and many loose guards of exactly the same design) you can see an imprint of pommelcap of the Vieuxville type.
This type, was found in the whole limes region from the Netherlands to the Danube delta. Mainly in burgi and castles.
So we have a roman pommelcap on a roman blade, and the piece in between has no features that mark it as germanic.
I find it reasonable to believe it is also most certainly roman.
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another horn hilt with vieuxville pommelcap from Nydam
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a sketch from a pommelcap Type Vieuxville on a horn guard from Nydam