The listing, Sergeant Preston of the Yukon 1950/1956 Trading Card #6 has ended.
Card #6. Mounted Police Headquarters.
These cards were issued first in 1950 to promote the radio series, and were marked "Challenge of the Yukon, Inc." and/or showing the 1950 date. They were reprinted in 1956 as an adjunct to the television series, and were marked "Copyright, 1956, by Sergeant Preston of the Yukon, Inc." They were originally distributed most commonly with cereal products.
Text from the checklist:
http://www.nslists.com/sgtprest.htm