4th century Saxons on Hadrians Wall?

[Flavivs Aetivs]

That's actually a good hypothesis, if there were Frisii guarding on Hadrian's wall they would have likely left with Roman troops back to the area of Kent and Flanders where they were settled.

I've never advocated true "migration theory." I always thought that the Barbaian Migrations were primarily where Barbarian cultures began to come in where Roman Authority collapsed. Post 454-Noricum is a prime example, when the Garrisons began to be pulled, disband, or be attacked by Barbarians because of Aetius' death, many of the Towns either surrendered to barbarians who offered them better protection, and were assimilated into the Alamannic and Rugi/Amali states.

Lauriacum probably being the best of these examples.

BTW where is Brigetio?