I believe that the majority of the 0riginal (19th Century) 'Ordnance' factories were closed down after WW1 as we didn't need to 'arm' as that was the war to end all wars.
In the late 30's, during and after the war 44 new 'Ordnance factories' were opened manufacturing everything from explosives, to Small Arms, to Ammunition, bombs etc.
It was these that became the RoF's. Any pre WW2 facility retained whatever name it was known by.
Most were closing down by the early 50's and in the '80s were 'sold off' and privatised and a company called 'Royal Ordnance Plc' was formed, later taken over by BritishAerospace in '87 and subsequently all but one of the RoF's closed down, the remaining one still manufacturing explosives which it sells to the MoD and export customers.
Many of the old RoF sites have been demolished (others are shopping centres or industrial parks), a few years ago I saw an advert for Lee Enfield parts, I followed up on these and met a guy in a car-park in Yorkshire, he opened the back of his car and there was virtually everything needed to build up 'brand new' No4 Enfield's, I bought some bits and pieces from him but stopped short at buying any 'bodies' (actions)
It turned out that the guy lived in Maltby and that as the RoF site was being cleared he found a way into the cellars, which were full of 'parts', he seemed to have 'claimed' quite a large collection of bits & pieces (many, number 3 bolt heads !!!)