Yes, polarizing filters are always optionally available. Those for the UB 6 have a clip for easy mounting, while those for the UB 4 and UB 5 are to be directly slipped onto the trial frame.
Polarizing filters with a clip as well as those of the slip-on type are for lateral mounting and can be easily flipped to the side when not needed.
On the UB 3/UB 3+ the polarizing filter has a central mount and can be simply removed after use. All linear polarizing filters are reversible.
Circular polarizing filters are available both with a clip and in a slip-on version.
There are also circular polarizing filters available in trial lens size for use in a slot of the rear lens holder if one should be free.