My Unique Identifiers for print editions consist of:
1) the title of the text, e.g. Snorre Kongesagaer
2) the year of the edition
3) an identifier for the copy such as library or place purchased
4) the page number, or numbers if the image is of a page spread
5) a number which indicates granularity for images from the same page
Example:
Page with two illustrations:
SKng-1899-Oslo-013-01
SKng-1899-Oslo-013-02