The sale of the Floyd E Risvold Collection is now very close indeed, with the live auction taking place over four days: January 23-26. Here's a look at another unique highlight:

It's often said that rare stamps are the most valuable commodity by weight in existence - more than worth their weight in gold. But rarely does this seem such an apt comparison as in the case of a Canadian cover sent to Rye, New York.

The cover combines a Seattle, Wash./Dec 18, 1899 datestamp with a 2c Red and an oval violet Nugget Express/Dawson Y.T./Canada stamp, with 'waving flag' cancellation on the front with a magenta double circle Nugget Express/Dec 18, 1899 stamp on the reverse.

Nugget Express cover

The Nugget Express operated out of Dawson City in 1899 following the discovery of gold in the Dominican River 50 miles to the south-west.

The combination of stamps on the cover is thought to be completely unique - no other cover has more than two of them - so a few days sifting at the New York auction could prove well worth the while of a philatelic prospector.