Thanks for posting, @Denny_Mark.
Based on what you've described and the advice already provided by the Adobe Community Experts, it appears your registered copy of Production Premium CS6 is an upgrade license. When an upgrade version is installed, Adobe will prompt for both the CS6 upgrade serial number and the serial number of the qualifying product used for the upgrade purchase.
The second serial number prompt is not related to Encore itself. You'll need to enter the serial number for the earlier qualifying product that your CS6 upgrade was based on, as others have suggested. Since your Production Premium CS6 upgrade was registered in 2012, reviewing your registered products may help identify the qualifying version.
Please see the following announcement for information on locating your registered serial numbers and the necessary installation files:
Download non-subscription Adobe software: https://community.adobe.com/announcements-616/download-non-subscription-adobe-software-554002
I hope this helps. If you have questions after reviewing your registered products and serial numbers, please let us know, @Denny_Mark.