A quick Google search leads me to believe these were made by a company called "Q Power." They appear to be a budget brand and your example looks more than a few years old, so I think it's unlikely you will find any documentation on them. From what I've seen with budget audio products, it's very likely there never was any.
Yeah, that's all I found too, so at this point it's either time to measure or assume it's a typical 8" mobile audio 'sub' for sealed box sizing or tune to 'taste' for vented once its Fs is ~determined using pink noise or frequency sweep.