Converted in 1972, it was used in the APT-P Power Car Test Train operating out of Derby, as such it had one end modified to couple directly to the Power Car, as did all of the Test Service Cars.

The STL file for Test Car 5, 8 and 9 is downloadable from here to print it yourself as many times as you want:

https://cults3d.com/en/3d-model/game/br-british-rail-3-different-mk1-apt-support-coaches

My unpainted kit includes Body, Chassis,4x Oval Buffers and a pair of B5 Bogies. Transfers are available from RailTec Transfers (2745).