The Irmscher panels were were the only ones to carry full colour-coding - which sort of introduced the idea on Vauxhalls of full colour-coding, which would appear as of Vectra B facelift etc.. (if we ignore stuff like the Manta in the 80s, which pre-dates everything
)
I would imagine that there would be
Irmscher stamped on there somewhere. Unless you've simply missed it sounds like you're given it a good going over, though) or for whatever reason, it's not actually marked. All Omegas are made by Recaro, but you try and find it written on there anywhere visible.
The normal PFL bumper is is several sections, the two main ones being the painted element in the middle and the large black textured part which this clips onto. Together it appears as one single component with bare black plastic top and bottom.
The only other option is a copied part, so someone's taken a mould off in Poland or wherever and made an Irmscher copy. It does happen, but you've already said the fit/finish is very good, so unlikely.
I say Irmscher!
PS - what's the date code, please?