No firm ID yet . . . .but now we know the source (
Nightjar was right!).
Roy Stuart's Glimpse 2 (1992)
Code:
https://www.roystuart.net/glimpse-2-vod
https://www.imdb.com/title/tt2437594/?ref_=ttfc_fc_tt
videos/1928095/roy-stuart-glimpse-2
The "mystery" scene begins about 3 minutes into this 2 hour + film, and has a bi-sexual masturbation theme. The imdb Cast List is not much help, but perhaps somebody will recognize her from other Roy Stuart material.
Note that "Mystery" has a mole/spot on her right breast.