Hey Carl. Yes..the story of the Abarth X1/9 Prototipo is quite fascinating.
If you would like to know more about them, I would suggest to start here:
Post all your pictures of the Abarth Prototipo/Stradale here.
xwebforums.com
Grab a beverage and sit in front of the biggest computer screen you have. That way you should be able to read the screenshots of the many articles contained in it.
Basically...yes the Abarth X1/9 was intended to replace the 124 Abarth for rally. Abarth made 4 or 5 of them for testing purposes in various configurations. At the time they called them "prototipos" ( Italian for prototype ) And entered them in European events where homologation was not required.
Back in the day - 1975 or so - to enter in the big rallies ( like the European Championship and the World Rally Championship - the WRC events ) you could not just build and modify any car you chose ( or make one from scratch ). It had to be a "production" car from a real car company. with only certain limited modifications allowed. The big loophole back then was that for Group 4 ( the forerunner of Group B ) a run of 500 cars was considered a "production" car.
So.....Fiat...in collaboration with Abarth....planned to make a run of special X1/9 Abarths. Basically...an X1/9 shell with big flares, snorkel, and spoilers. And a twin cam 124 based engine of 1850cc with special Abarth heads and twin carbs. Special suspension and brakes, Modified Beta/Scorpion transmission with LSD. Etc, etc.
After testing, Abarth and Fiat finalized a specification for the official run of 500 cars. And made one only car to that spec. Known now as the "Stradale" ( Italian for street ) version. All 500 cars were to be made to that exact spec. And would be available for purchase as a high performance street car - but likely only in a couple of European countries who would allow such things.
Those wishing to convert them for rally use could then further modify them under the allowable "modifications from stock" allowed by the rally rules at the time. For example, fender flares could be only 2" wider than "stock". So the Stradale flairs were smaller than the rally spec flares. Stuff like that.
So the story goes.....Abarth completed all the tooling and plans for a run of 500 cars. The homologation papers were all prepared, using the one only "Stradale" version for specs.....
At this point.....the suits at Fiat HQ decided that Fiat should instead use the 131 ( a much more important car for Fiat sales ) as the basis for a new rally car instead. and thus the project was shelved at the last minute. Ah...what might have been.....
It is said the 4 or 5 "Prototipo" versions are still around. Abarth stripped them of their drivetrains ( there was a shortage of the Abarth engines at the time ) and sold them off. Legend has it that Al Cosentino managed to get 2 of them. It is said one is now in Italy and one in Japan or something.
And the one and only "Stradale" version ever made.....well apparently Al also managed to get that too. It is on the cover is his "Faza Bible". It ended up in Australia with a guy who bought his Faza empire. It sat there for many years and was finally restored back to its original spec last year ( Al had messed with it quite a bit ). And it even appeared at a car show in Australia not long ago. There are videos of it at the show. I'll try to find them.