Later Roman arm guard?

[Renatus]

Actually I was already quite confused by this sculpture! Is it believed to be a third century piece reused in the reconstruction of the city of Civitas Tropaensium by Constantine and/or Licinius, or a fourth century copy of a third century piece, or a third or fourth century copy of the original trophy on the top of the Trajanic monument?

I glanced at D'Amato's book in Waterstones this afternoon and, as far as I remember, it says something about the sculpture being "retrospective", which could mean that it is third century piece re-used or a fourth century copy of a third century original. My initial thought was that it was the latter but now I'm not so sure. And why use it as a model for a centurion anyway?