BMC and Leyland inlet valve thimble. This thimble holds the anti-rotation key in posistion to stop the masked inlet valve from turning.
Note the mask is to swirl the incoming air used on Nuffield.
See item number 18 in the illustration.
Used on engines:
Leyland 4/98DT up to engine serial number 13804
Leyland 4/98NT up to engine serial number 83027
All BMC 3.8 engines
BMC 3.4 from serial number 18824
All BMC 2.8 engines