This cache IS at the posted coordinates. You may sign the log at any time, however DO NOT log it as "Found" online until you qualify.
Souvenirs are awarded for U.S. State, Canadian Provinces, Countries, and special occasions/locations. You must be awarded 50 of these to qualify. If you log it as found and you DO NOT qualify your log WILL be deleted. I'm sorry but it's simply not fair to those that DO qualify.
Use your "life list" of Souvenirs. There is no logical way to determine a "starting date" for this type of challenge. The evidence is in the "Souvenirs" tab of your caching profile. It really is that simple. There is NO NEED to verify your "list" before going to find the cache.
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