Peter Kerslake says "Railcar number 17 was the last built by the Gloucester Railway carriage and Wagon Company and differed from their earlier twelve units in that it was turned out for use just as an express parcels car in 1936 with no passenger seating capacity and intended by the Great Western for use in and around the London area.
The railcar looks rather drab and unattractive in this view especially with so many of her side valances removed and the unsightly ventilation panels all exposed to the elements.
There was a second "parcels only" railcar numbered 34 put into service in 1941 but bodied by Swindon Works and my 1956 shedbook has number 17 seen here allocated to Leamington at that time, with number 34 being at Southall."

5808 in the background was a long term Oxford inmate, being withdrawn in February 1958 which helps date the photo.