Phenjan,	Changchun DK4	— 097
Kim Jong Thae DK4		— 231

This image shows a 4 axle trailer car, most likely built by Kim Jong Tae locomotive works, which is different from the DK4 as it has no air conditioning, and has no cab on either side. The side fluting also appears higher.
These cars were most likely built to expand the DK4 from a 2 car set into 3 and 4 car sets.

source: https://gamp.ameblo.jp/venus1969/entry-12427078884.html