Hi, I could do with some quick views to advise a UK GLAM institution as to how to make part of their digital archives of images "No copyright known", based on the date of creation of works (a proportion may have year of publication instead). To make this an easy step for the GLAM, there needs to be some evidence of why any date chosen is no risk, or extremely low risk, so that a case could be made to the institution's management; and lawyers.
I'm aware that Google Books ran into a lot of lawsuits for releasing book scans a bit too liberally, while at the British Library there was a very conservative date of 1874 (I think) where everything before than could be considered public domain as a default.
The tricky part is to choose a generic date that can apply regardless of the nature of the item digitized, or its country of creation/publication.
I have created a thread on-wiki at - https://commons.wikimedia.org/wiki/Commons:Village_pump#For_an_institution.2... - any links or advice from publications would be greatly appreciated.
Thanks, Fae