I don't know anything about the Polyethylene enclosure you mention for the Nucleus model that IIRC was available alongside the Solos circa mid to late 1990s. Same concept / design, just with an additional woofer sphere and higher in the Gallo lineup. Never seen the poly in person nor in a photo, and I've never heard of it nor read about it.
Both my pair of Nucleus Ref 2s (the 2 ball model) had aluminum spheres. As did another local pair of Ref 2s that I was in the same room with.
And like Zerojeff, there are times that I too regret selling mine.
Well, there's a photo of the poly enclosure Solo in my earlier post on this thread. Note that you call the speakers "Ref 2s", the 2 was for the second iteration, not 2 spheres. I have the original add flyers form both the early and second gen Gallo speakers.
I purchased my early versions directly from Anthony's basement in Brooklyn in '92 or 3. I had my references upgraded to Ref 2s at his modest factory in Brooklyn in '95 or 6, as things were going well, and he'd moved out of his basement. I can try to scan the flyers and post them here. I may have some more personal pictures as well, but as that was pre digital era photography, I may have difficulty putting my hands on the pix.
P.S. The naming convention was Nucleus Solo (single sphere) and Nucleus Reference (2 spheres).
There was also a Nucleus Minor, that used a concentric MidWoofer-Tweeter (similar to many KEF speakers. I think it was actually a KEF driver).