HST is painted on the buffer beam. HST emergency couplings were a rigid bar with a lug which fitted over the drawhook of the assisting loco. I vaguely remember being told during training that some classes of loco could not be used to assist because there was too much or too little sideplay in their coupling hooks (I can't remember which now, sorry). I think the branding indicates locos modified so they could be used to assist.