Had a quick look and that's definitely a 4w PO sorting van. There were only about 10, (2962-2971) built c. 1957/8, and conceivably could have been turned out in the livery of the day, 'silver'. The silver livery would have been introduced on the A class as early as 1955, some 1429 series coaches c. 1956 and C class as late as 1958. The photo in Irish Metro-Vick Diesels is taken in 1960 which would fit that timeframe with the accompanying C class newly turned out in lighter green.
All the other photos of the 4w PO sorting vans that I have seen have been in Black and Tan which would have been introduced about 1961. There are some photos of other 4w vans e.g. 4w HLV turned out in green but unsure if I have seen green 4w sorting vans (except bogies TPOs), so any silver ones repainted directly into B'n'T, one supposes.
That's a great find, Frank!