Hidden Mickey Pins Question

I tried searching but couldn't find a definitive answer. I wondered when did they start making the hidden mickey pins available for sale? (the mystery bags) I ask because in my research for some pins I want it seems some older hidden mickeys are selling for more than later styles. Like this mad hatter pin- http://gallery.pinpics.com/cgi-bin/pin.cgi?pin=40555&sid=8972.1351729100.v2 I thought since they were not LE they didnt hold much value and most seem common but my best guess its the older ones are worth more because they didn't start releasing them in the mystery bags until later dates? Or is there only certain sets released to the mystery bags? Im also wondering....are the mystery bags just whatever is lefover from the entire run of those pins? Or do they purposely make x number of pins and set a number to give to cast members with the remainder being sold in the mystery bags? Still learning all about this, and I love the history of things! And theres sooo much to learn!!

A cast member told me they produced too many of the newer HM pins so they started selling them in mystery bags.

Not sure, but since I've been collecting, I've seen them being sold. The earliest one I remember being sold is the Robin Hood coins.