Note that if ordering stems only, they are the 'late' (850 Mk3 and later replacement) stems/assemblies with stepped outer threaded section with two flats (that prevents the housing rotating).
I don't think the lamp housings are interchangeable between 'early' and 'late' stems and the fittings are different as the late/replacement type has a retaining washer (also with flats) instead of a nut on the outside of the housing and different internal nut.
https://andover-norton.co.uk/en/shop-details/17974