How does a double-layered Vacuum Fluorescent Display work?

[GnatGoSplat]

Came across this random auction, I think it's a car stereo with built-in EQ.  What caught my eye that it's got a "DOUBLE LAYERED / FLUORESCENT DISPLAY".  Looks like it has a transparent VFD on top of another VFD.
https://www.ebay.com/itm/133869488338

Also found a Youtube vid.


So that got me wondering, how did they pull this off?  The phosphor segments are usually visible, how did they make them transparent?  Were there any other devices that used a double layer VFD like this?

I've seen see-through OLEDs before, but first time to see a VFD like that.

[tooki]

Presumably it's simply some kind of transparent phosphor. They certainly seem to exist (see the images on p.216 of this journal article).

Or maybe it's ordinary phosphor, but deposited as a pattern that is sorta see-through, kinda how we see through window screens?

Either way, it'll certainly use ordinary indium tin oxide as the transparent conductors on the glass.

P.S. I can't access all of them, but there are journal articles and patent filings about transparent phosphors, e.g.:
https://patents.google.com/patent/US2980550
https://opg.optica.org/josa/abstract.cfm?uri=josa-45-7-493
https://ieeexplore.ieee.org/abstract/document/7267309
https://opg.optica.org/josa/abstract.cfm?uri=josa-47-4-300
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6422321/