in quest of the best web sites for collectors
Stamps on Stamps
SOSIG (Stamps on Stamps Italian Group)
The site has a checklist of stamps on stamps (with Michel and Y&T catalog numbers).
Stamps on Stamps
Stamps on Stamps Collectors Club
The Stamps on Stamps Collectors Club is a non-profit philatelic organization. Membership is open to anyone interested in stamps on stamps, stamp anniversaries, and related topics. The club's bulletin is SOS SIGNAL, which is issued quarterly and contains new issue information, articles of interest, and news about the topic. Some informative articles from the bulletin are included on the website.
Overprinted British Stamps on Stamps
A page of GB overprints on stamps on stamps.