Yes, I believe they are the old S-cards. As you noted, this is not an MPEG4 issue.
I wish I could say how they fixed it but I got on the phone with Verizon who ended up working with "Level 3" support to do something that got both tuners working fine with the free preview. I had to read out the cable card info multiple times.
So it appears the channel 131 test to see if cablecards are paired correctly does not work in all cases...