how can it not be?

it states to cover:
CD-4CF, CD-6CF, CD-8CF, CD-8 BUBBLER MODELS
basically everything with the sybersonic computer system (part SV-70003-2A) in the nostalgic series.
if your unit does not have the "sybersonic computer system" then perhaps someone has swapped out the entire system for something else and you'll have to find out what it was from.