I've been meaning to share this info for months now since I spoke to one of the very helpful guys at Power Brakes SA.
https://www.powerbrakes.com.au/brake-component-restorersTurns out they have the ability to completely rebuild the all alloy VH24 Bendix boosters used in early Holdens up to EH. The diaphragms have been almost unobtainable over the last few years so they have found a replacement that works and can rebuild the whole thing for a bit over $600 (plus freight both ways. Still much, much cheaper than the ebay based alternatives and all you need is a decent booster to send them - no exchange.
Worth sussing out if you are wanting to go all original under the bonnet.
They are very polite and really know their stuff.
Trust this helps someone.
Dave Mac