This is my first post after joining this forum.

Let me begin with a question out of my curiosity on mixed covers of joint issues.
This is a mixed first day cover of the U.S.-Italy joint issue of 500th anniversary of discover of America 1992 (Zimmerman's Joint Issue Catalogue number 19920424). I noticed that on both Brookman and
Scott's First Day Cover catalogues, this joint dual cancel with "blue
cancel" is valued at US$450. However, Zimmerman's latest Joint Issue catalogue did not mention this item. I also found two more covers that has the same "blue cancels", but are not mixed covers. Does anyone know more about the storied behind these "blue cancels"? and why is this particular cover listed with such high price?

