My V50 III was missing its seat lock when I bought it. Presumably a previous owner had lost the key and removed it.
This is the type of lock that fits in a cylindrical lug on the frame, and pulls a long hairpin spring to release the pin lock on the seat. I think it’s the same on the V35 II and maybe V65 models.
Needless to say, the parts are no longer available.
After measuring up the hole and figuring that it was probably a lock of Italian make, I found that the Zadi locks common on caravans and the like have the right barrel diameter to fit.
This item here, a Zadi Vecam 13mm barrel lock with 34mm length fits right in:
https://www.ebay.co.uk/itm/184442134955 Cost about £7.50
Link to Zadi site: https://www.zadi.com/en/caravan-en
I had to do a little filing to the rim of the frame lug to locate with the lock boss if I remember correctly, and make a simple offset crank plate to go on the end of the barrel and pull the spring (another unavailable part), but all is now working well and similar to the factory arrangement. I hope this helps somebody with a broken or missing seat lock!