Well, you learn something new every day!
It appears there are 2 different types of rear shock on the BIS according to Fiat ePer:
LINK 7591373 – tapered bottom bushes
7595878 – cylindrical bottom bushes (this one was on my 126)
The ones Ricambio have in stock are the tapered bushes:
LINKRicambio also sell the tapered type of shock here:
LINKSo I suppose I'll be buying a pair of those type of shocks then!
Even @gadge didn't know this!