I once spent a summer in Oxford at Worcester College, studying Greco-Roman Art. I had a reader's card for the Ashmolean, but never saw those drawings! I have been able to see some fabulous sanguines here in the US, though, Watteau being a favorite.
I can't take much credit for this. Now, If I could figure out how to get the look of silverpoint with white chalk highlights, I'd be proud of myself!