Shimmers are nice to work with, but what did they use last time?
Do you have swatches from Union or Wilflex that may help you determine what you're looking at?
Real metallics have a distinctly different look than the plastic shimmers when examined.
Either way, it's probably over a Poly UB, and as you learned, Poly can give you a very smooth print.
edited for a bunch of rushed spelling errors lol!