David,
That's my question... lol I can't find any reference to this specific series at all.
I believe Pamp Suisse, Engelhard, and Johnson Matthey, respectively, have the highest premiums and demand as investment 'collectibles' because of several reasons including, but not limited to, quality, rarity, and diversity.
Their high premiums tend to exclude them for straight bullion buyers who invest and depend on future precious metal prices to increase in order to profit. On the other hand, proven 'collectible' bullion has kind of a built-in safety-catch, if silver prices plummet, they still have the 'Collectible' value as a back-up, of course if they want to profit from rising silver prices alone, they would have to wait much longer, until their costs (including premiums) are surpassed. Two completely different perspectives.