As found on service sheet 374 for various B1 locos, however being black I think they are only designed for R2998.
Hornby Genuine Spares X9075W Cylinder Valve Block For Black 5 Loco For R2258 5MT
Hornby Genuine X9074 Cylinder Valve Block Black 5 For R2250 R2359 R2322 R2449
