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.
MGOC annual membership currently cost a little over £40. See: https://www.mgownersclub.co.uk/benefits
In addition to the shop, Two-Sixties full members have unrestricted access to a wealth of technical information and personal advice from expert owners here. To become a full member, send a message here to Tim, with your email address, asking for a questionnaire.
Hope that helps .
David