>So, they do work, they are just not supported officially by cisco, correct?
My understanding is that it *sometimes* works, and sometimes doesn't. It's not that it works intermittently, it's that some hardware+software versions will work fine and some will just not support it. This is second-hand information, so apply an appropriate amount of skepticism. Cisco's official line is clearly that the first-gen T1 CSU WICs are "not supported" on the ISRs.
Do also keep in mind that eBay is flooded with counterfeit first-gen T1 CSU WICs. Your odds of getting a genuine part cheap on eBay are not good (and eBay is doing it's usual nothing about the problem). Part of Cisco's motivation here is to just kill those off. The clones are fine for home lab use, not for production use.
I don't read the chart you link to as confirming what you think it confirms. The 2620 platform is not listed anywhere on the compatibility matrix.