By the mid 1950's, many passenger trains were still screw coupled. The new diesels in service were equipped with auto couplers only. A group of vans were fitted with steam loco swivel couplers which allowed the vans to act as transition vehicles between the engine and screw coupled carriages.
Railway records do not date the modifications. To indicate the dual coupling arrangement, the vans remained coded as _Z__ but were were lettered with a 'P' in the corner to denote 'Plural' couplings. The vans were ballasted with 3 tons of rail, no doubt to improve the riding quality. The ballast was lengths of rail along the floor with a false floor covering the rails.
From 1956 the vans were recoded to ZP. Despite the 'P' in the corners, the vans were only authorised for a maximum speed of 60mph ( WTT Addenda 28/6/1971 ).
With limited traffic in the 1970's, the vans had the ballast removed, speed derated and were recoded to _ZD_.
The ZP numbers were:
19, 31, 33, 56, 85, 153, 181, 182, 184
187, 190, 194, 197, 198, 206, 211, 212, 215
222, 224, 227, 237, 239, 240, 253, 260,
468 - 471, 494, 498 - 500, 585, 599 - 601, 603
604, 611, 619