RX1900, Sorry but I'll disagree with you.
I have the original Fiat microfiche for my '79 X and it shows two different bushings (front side and rear side of the front pivot mount or "support") with two different part numbers (for the same side of the vehicle). Unfortunately I have not converted the fiche to a digital copy yet so I don't have a way to upload it here, and I'll have to get to the local library to use their fiche viewer and read the actual part numbers again. So for now I cannot validate what I'm saying, but I will when I can.
I suppose it might be different for various years of X? Perhaps Fiat used two different bushings at some point but not for all years? And I understand why you would think they are the same, as all of the vendors list them that way. But according to my factory parts fiche that is not the case. I imagine the smaller bushing is no longer available (I was not able to find any new ones when I searched that part number), so that is why the large one is used for both sides now.
To add to my view of them being different, when I removed the originals from my very low mileage X (with mostly original parts remaining throughout the vehicle), the two bushings were indeed very different in dimension and shape. I might still have the old ones (not sure if I kept them or not), but if I find them I'll post pictures to show the obvious difference between them.
If you look at the picture Jeff S. shows in his earlier post above, you can see #4 and #8 are visually different (as they are on my fiche); #4 is described as "rubber pad" and #8 is described as "rubber ring". This also tends to support what I am saying that they are different. But I realize Fiat wasn't always that accurate on such matters, as you point out. Therefore I will concede the information on my factory fiche might also be incorrect (despite having two different part numbers). The fiche I have also includes a later page showing updates to part numbers, i.e. when a new part number supersedes an old one. And I don't recall if I checked that page for these two numbers...maybe it already shows both bushings under the same part number (e.g. perhaps they subsequently replaced the small bushing with the large one)? I'll check for that also when I can get access to the fiche reader again. But my original old bushings were obviously different when removed, so when they were installed at the factory they were two different parts.
To your point about #8 in Jeff's diagram being a grommet for the support bracket and not a rubber bushing. My '79 X did not have such a grommet on either support bracket (left or right sides). Again, my X is very original (unfortunately to the point the original owner did very little maintenance or servicing to it), so I believe this is how it was assembled at the factory (naturally there is no proof of that). And my fiche does show both cup washers, bushings, etc. on either side the bracket (unlike the illustration above). Maybe this is another change at some point of the X's evolution? Perhaps different assembly arrangements were done at various production runs depending on what parts were available at the time (not uncommon for Fiat)? Who knows. I'm curious what year(s) of X is your information from?
I imagine there could be other explanations as well, but until I here them I'll stick with what I have said. Not that it matters, I am not trying to prove right and wrong, I'm just adding clarification to what I posted earlier. My intent when posting this was to assist Jeff S. with finding the needed parts; it was a bit of a difficult decision for me when I could not find the smaller bushing and wasn't sure if the large one would work well on both sides. But effectively that is what I ended up doing (however my X is still under construction so I have not road tested this yet).
Thanks for the input, Jeff