Thanks, Bill! Glad to help.
Electrolux Corporation's filtration initially included an exhaust filter (Model XII to the early Model E - Automatics). Around 1958, it was discontinued as being unnecessary with the new multi-ply paper filter bags.
In 1985 when Electrolux Corporation and Electrolux Canada, Inc. merged, Canada's Lexan tanks (Hi-Tech 2100, L-E 2100--later 1623) were added to the U.S. line and re-introduced the afterfilters to the American market (Canadian cleaners have all used them). None of the metal cleaners of the U.S. line from 1958 to 1993 used them.
As for the pre-motor filters, according to the users manuals I've seen, ProTeam, which partnered with Aerus on designing and manufacturing the Electrolux Commercial upright vacuum cleaners, introduced them to Aerus (Electrolux USA) customers in the late 1990s. Interestingly, the Lux Legacy owners manual in the "current" manual menu on Aerus's website doesn't include a pre-motor filter so it must be an older one.
I'm not sure about the Model XXX pre-motor filter you saw. I have several Model XXXs, none of which have one.
You can get some filter material, though, and cut your own to fit the bag chamber of your HPR.