It might not be the same fitting on yours but image below is link to the retaining clip used on the Lucas PRS 3 ignition/light switch of that era.
![[Linked Image]](https://i.ebayimg.com/images/g/Rm0AAOSwrhFeJxLh/s-l1600.jpg)
If your switch body has similar grooves for retaining clip the above should solve your problem. Failing that complete generic PRS 3 switches as used on many cars and Land Rovers in the 50's and 60's are occasionally available on eBay. Although there are a few style variants the wiring remains much the same between them accepting that later ones have lucar spade connectors.