Good morning Brian and welcome to the Forum.
The Black Olive valve you refer to is only available through the club shop, open only to full members of the Two-Sixties.
Quite a few special 260 items are available there, many commissioned and made specially for club members.
With parts become scarce or totally non-available, the club shop is often the only way of acquiring some items.
The Two-Sixties Forum is hosted by the MG Owners Club, and MGOC members can join the Two-Sixties as full members, free of charge.
To become a full Two-Sixties member, send a message here to Tim, with your email address, asking for a questionnaire.
Hope that helps
.
David