Hehe, pretty crazy how much they can do with a tiny core like that! I always knew CableCARD was pretty crazy. I have a few of those in some TiVos.

Looks like Digi-Key has the MX29LV040 chip. It has a C suffix that CC's card didn't have, but it returns the same chip ID. I'll give the Macronix chip a try, and I'll also grab a few more SST39VF040 chips to make sure I don't just have a weird dud.

http://www.digikey.com/product-detail/en/MX29LV040CQI-70G/1092-1126-ND/2744809