I've done a very limited amount of poking about at a few museums that might have inherited a painting from "Prince Luitpold of Bavaria," but although there are a few dozen possible ones to check, they semm not generally to have posted indexes of their holdings. The few that might have "searchable" postings on the web all appear to use "fuzzy search" so that when you enter "Prinet" as a search term you get all the results that might include "Prinet" OR "Print" OR "Parent" (all three contract to prnt for typical fuzzy search) so any possible result is buried among lists of every piece of paper one can buy from the museum gift shop.

There's also the slight handicap that most hits for any museum will be from travel agencies who's only info is that they can get you a ticket to get there. It's not always evident that they actually know where "there" is - but "they'll look it up" if want buy the ticket.

John