Hi Hal, I would attribute the condition to it being housed in the glassine envelope for the last hundred years.
When people first started making there own postcards or personalizing post cards they used glue and glitter among other things and this caused the post office to require such postcards be housed in a glassine envelope.
Correct me if I'm wrong but I believe it was around 1910-1913.
Here"s a good example of one with the glue and glitter that was required to be in a glassine envelope.
Note that this envelope deteriorated over time and the postcard yellowed both of these postcards were stored in the exact same way.
Cheers, Bill


