BTW the car is a 1967, I was told it was an F. Sorry I said in earlier post it was an R. So if it's a 67 it must be an F right ???( it has front hinges ,round binacle) but a 600cc engine
It's all been said in parts by others; I'm no "expert" but the simple "fact"? that I've learned from those others and experience is that later Fiat 500s, possibly starting with some F's and L's and then (I'll wager) certainly into the R's, were fitted with what are commonly know as 25mm driveshafts but are sometimes listed (maybe more accurately) as 24mm.
The 126 continued with the 24mm diameter but had to be slightly longer because of the dimensions and geometry of the swing-arms.
So if your gearbox comes from a 126 and hasn't been modified, you need to replace the driveshafts with ones listed as suitable for a 500. It will still work with 19mm shafts but would be unnecessarly under-engineered
If it has already been running with the 600 engine the shafts are likely already to be correct.
Your original post says the gearbox is stamped 110F... From my experience (non-expert

) that might mean that the innards of the gearbox are those from the F/L range of cars because the gearboxes I have seen, which are of 126 heritage, all have 126AB stamped on various castings.. To be marked 110F might be the norm on the box of an original 500R and might explain the non-sychromesh thing despite the contemporary 126 having sychro. But it could be that someone has grafted an 500 box onto a 126 bellhousing.
Anyway, regardless of how much the previous ramblings ,might prove to be conjectural...you definitely need the shafts and matching drive-flanges labelled as 25mm/ 500 type.
