Most probably. Mine is 20 digits long as well. If they added this in list just for sites which provides Steam keys for Call of Duty 4, i don't know. They don't add most of retail games on that list but, in the end, it's a well-known franchise, so maybe...
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According to this renowned official page, you can add Call of Duty 4 boxed copies to your Steam account.
However, that seems like it's still broken. I've tried both original format and divide-into-five-characters format; Steam says that key is invalid. Anyway, feel free to try if you have a boxed copy.
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