Hi Cyrus,
Yes - we quickly identified that it was the same date as the Panda 20/EMD update.
I've posted on here several times before in relation to it but it has always appeared that what we were doing was a good thing to try. Out of a working relationship on another site, I've also got a good relationship with some of the people at SEO Gadget and have often ran ideas past them.
Being an eCommerce site, we had a large number of pages with snippets of duplicated content (in our industry the product, which is phone unlocking, doesn't change - only the phone that is being unlocked). That meant that for very old phones or ones that weren't really important, there was no point creating content (we do an under H1 paragraph, offer unique instructions, provide a video of us unlocking, etc) for it so we'd include the delivery time and other important information a customer would need, which was the same or similar on a lot of pages, but importantly necessary.
When Panda 20 hit we had around 10,000 pages noindexed and only indexed those that we had created unique content for. This was on a .co.uk version of the site. After a year of not ranking, we then had to buy and move to a .com version, without redirecting the old site as we didn't want to take the "penalty" with us. We changed a handful of important links and took the design (which we always A/B test on) with us but essentially started anew.
We've tried blocking the bots altogether from visiting the pages we knew shouldn't rank and are now trying an old method of ours, which inclues a # in the URL for the vast majority of pages. We've left only a handful of phones with a full URL to see if they rank but no luck so far.
The thing that really miffs me is that the "iPhone 5S Price Index" page we've created ranks number 1 in Bing and doesn't exist to Google. Yet all of the results link to us. We also rank 5th for one of the keywords we use to, which shows we don't have a complete penalty on us.
Have you, or anyone, ever seen this before? It really makes no sense that we rank for "unlock mobile" but as soon as you add "phone", we again don't exist.
BTW - we did do an audit just before Christmas, a few changes here and there but nothing major. I honestly believe this is one of the most advanced penalties an SEO can work on.
Thanks.