Genuine plastic cap as supplied on single line brake cylinders pre 1976 and all clutch masters.
It doesn't often break or wear out but it gets lost or turns yellow. The early steel master cylinder reservoir caps had a tendency to rust on, so this white plastic could be considered an upgrade unless you wish to keep your Pre 1963 car original with a metal cap
17H3723 alsofits Midget and sprite and MGB 1963-79