I see a handful of listings on Ebay and mentions on other forums of people supposing Yamaha IC IG00155 is a suitable replacement for the filter IC IG00156 in the CS series. This is a warning that it is not, at least not without modifications to the voice boards. If anyone has different experience with them please let us know but we tried it and it does not work as a drop in replacement.
Thank you for posting this. If you discover any more please add it to this.
cheers