Im leaning towards the same conclusion. According to the serial data out on the webs, this is all pointing to a 1958 era 500N. (58 falling between the
028539 to 051383 vin range) with the fender stamping on yours showing 110. 044581. typically on the D or F series cars it would be stammped 110D, or 110F before the Vin in the sheet metal.
With that said, it does not have the N series front paneling, It definitely had been replaced at some time in its life with a L or R front facia, and L/R badging. A N/D/F will have the classic fiat mini grill crest with mustache pcs. And a circular hole behind the mini grill where the horn is typically housed. Again, this is just my observation from the photos.
The Hardtop seems to be one from a D. When they made full rag top cars, along with cars with a bolt in clamshell, and conventional fabric "short" sunroof. I dont know if the early N's were ever offered with a hard shell top, most of the ones ive seen had full rag top. With that said, it should be removable (bolt in), and you could return it back to its full rag top glory.
Edit: looks like the later variant N's came in both variants. Dont know about 58.
You have yourself a early model car if the vin is correct. Down the road, i would recommend you consider restoring this N back to its formal original self, the N's are very collectable.
Like Goldnrust mentioned above, if by chance the engine is still original, the stamping will help with its originality. N's had a 479cc engine, while the D's had the 499cc
This could potentially be one of the NewZeland built 500's. which could possibly be a mishmash of parts. I unfortunetly dont have much info on the specs of the cars built there. I do have the Bambina book, maybe it would show some insight.
I dont know if any of the early model N's were ever offered in RHD. to the UK market. Guys any insight on that?