[Old Davy]

Thanks for that, steamer99.
And to everyone for your valued input.
It's easy enough to guess but better to actually know.
Between this group and SVX World Forums the true answer to this niggling question is emerging.

Here's the thing - although my '97 SVX has all original body stickers intact, the underhood Emissions Declaration makes no mention of OBD.
But, it has the two black OBD connectors under the kick panel as well as the 16 pin OBDII socket in the dash under the flip up door.
I took possession of the car with the engine light constantly on with no effect on performance. The CEL would go out again for short periods during gentle city driving but came back on under spirited hiway runs.
Diagnosis at the dealership threw out the code for a right O2 sensor and the MAF. This after a lengthy and somewhat shifty/dubious explanation that the shop had to use 'outdated' OBDI software. They were, of course, only too eager to change those parts for me at great expense. :-) Not gonna happen since the engine is running top-notch as is.

Back home in my own driveway, armed with intuition and the wealth of information from the site, changing the green cone intake filter back to the OEM airbox & paper filter, and giving the MAF a good cleaning of K&N oil contamination, the CEL has stayed off the past week. I believe the EGU is gratefully relearning to run on 100% 94 and 91 octane rather than the 89 mix it was getting before.
I intend to keep this car for the rest of my life. Due to its relative rarity up here in Canada and gradual natural withdrawal of support from Subaru North America I have been gathering every scrap of technical information I can find off the web and sticking it all in a 3-ring binder for myself and subsequent owners.
I didn't know squat about OBD but I'm learning fast. To this end I have located a good basic OBDII scanner reader through Amazon.ca (sure wish I had an address in the USA). I'm looking forward to getting my own readings should the CEL begin acting up again and having the ability to clear old codes/resetting the CEL without having to sip bad coffee at a Dealership.
After Labour Day when my work schedule drops from 60 hour weeks to 40 I'll get this Canadian car up on ramps and be able to determine if in fact I have a third O2 sensor to be concerned about.
As stated above - It's easy enough to guess but better to actually know.

First things first, tho. Next week the new AC Condenser rad gets installed and that will be very cool...