The Gemini will be much harder to find. The Hydra cap is same as a number of common wheels particularly the 740 turbo “Draco“ style. Berry Motor Group in Melbourne is having an alloy wheel clear-out at the moment and I know Mark said they have multiple sets of Dracos so they should have caps. The tabs do break on them sometimes. They are available new from various sources (for example Skandix or FCP Euro
http://www.skandix.de/en/spare-parts/suspension-steering/rims-wheels/wheel-center-cap-silver-for-genuine-light-alloy-rims-hydra-argo-draco-adhara-orion-piece/1009395/
https://www.fcpeuro.com/products/volvo-wheel-cover-740-760-780-940-960-1343663
