I used to identify the 850 Sport Coupe by the presence of a reverse light/lamp switch, I don't remember the Saloon or 850 Van/Familiare having a reverse lamp (I do seem to recall the 850 van had a lower final drive ratio inside the gearbox(.
There was a red or yellow spot of paint applied by Fiat to the upper center area of the bell housing or around the gearcase vent which would indicate it had an 8/39 (4.88) final drive gear set (ring & pinion) inside, as used on the Spider, Sport Spider, Racer, Coupe & Sport Coupe models.
Of course, this paint may be gone if vigorous cleaning of the transmission has been done. At that point, removing the bell housing & looking at the info stamped into the beveled edge of the ring gear is pretty much the only option left.
I used to identify the 850 Sport Coupe by the presence of a reverse light/lamp switch, I don't remember the Saloon or 850 Van/Familiare having a reverse lamp (I do seem to recall the 850 van had a lower final drive ratio inside the gearbox(.
The reverse switch was the giveaway for me too.
There's no difference in ratios between the coupe and Saloon is there? I've a couple box sitting in the garage and I'd do a swap if there were a few less rpms
I knew someone who rallied an 850 Coupe (back in 'the day'), he fitted a gearbox from an 850 van, which had a lower final drive ratio than the Saloon or Sport, iirc it did c. 80 mph @ 8,000 rpm in 4th gear (i.e. c.10 mph per 1000 rpm in top gear) - so you certainly don't want to use this 'box if you want lower revs for a given speed.