That's not just any oul 400 class, but one of the two Caprotti valve gear ones - it looks like no.406 with the straight footplate. This one lasted until 1957 - the photo suggests it had been out of use for a while, as corrosion is clearly visible on the reversing rod and the upper slidebar.
The other Caprotti 400, no.401, kept going until 1961 and may have used spares cannibalised from 406.