How about the first result for "museum cataloging software" in DuckDuckGo, CollectiveAccess? This was without adding an "open source" qualifier. It seems to be in use in the industry too.

https://www.collectiveaccess.org/ https://www.dublincore.org/groups/tools/dc2008/dc2008_seth_k...

Another one apparently in use in the UK is CollectionSpace:

https://collectionstrust.org.uk/software/collectionspace/ http://www.collectionspace.org/

Also:

https://omeka.org/s/ https://www.museumplanner.org/free-museum-collection-softwar...

Yes, there are plenty of them. That is not the point I was making.

Hmm, then your point wasn't clear.