I would argue that all the material held by any public funded Archives IS in the Public Domain.
The material held does not belong to the Archives it belongs to the Public.
People who donate material to public Archives surely do so to enable interested parties to have access to said material for their research and knowledge and by doing so they freely gave their possessions for the great good and that any public Archive does not have the right to put limitations on what use said material can and cannot be used for as long as it does not violate the wishes of the donor family.
Acknowledge the source of the repository of the material by all means
The Archives remit being the preserving and safeguarding documents and items freely donated into its care.
There are many collections held in archives which are accessible to the public which have restrictions placed on them, either by the depositor (eg, copyright, being asked before allowing an item to be looked at, and so on) or by various pieces of legislation, such as what is now GDPR. Many collections, including photographs, will have been deposited with no thought to copyright. Photographs in particular are a minefield, but so are things like orphan works and unpublished material. Copyright on some items that are classed as 'old' can last until 2039.
Material in public archives doesn't belong to the public. That's like saying houses or property owned by a public authority belongs to the public, which isn't true - they're for the use of the public, in the same way their archives are.