Thanks for the info. It's not planned to have the delta chd thing enabled by default.
I'm thinking about an optional information if a clone chd which is not yet a delta variant can be a delta variant and an optional convert (via chdman). Everything disabled by default. It's all in the planning at the moment...purely experimental. I wonder how future-stable such a conversion is anyhow....if a set changes its parent/clone relationship in the future for example...For MAME's load routine it is most likely not relevant since it simply looks everywhere for the beloning parent-chd.