It is the late Roy Amesbury's 3.5"g Brit, and Roy described it in ME in 1966 or 1967. A search of the Index will provide the relevant MEs.
I cannot recall from memory whether Roy explained the 'oil – no oil' gauge, but he later described for his later GWR loco making all his own pressure gauges around 1972 and a duplex brake valve gauge. He later described his GWR President loco around 1982 with his homemade gauges. Roy worked at Rolls Royce in Derby.
Hope the above is some interest.
Cheers,
Julian