Sorry it took so long to get back to you on this, too much going on in life, but actually the splenial bone in polypterus DOES have those small tooth like structures.
I haven't had time to find a purpose for them, especially since we established that they lie well beneath the flesh in the jaw, but that is definitely the splenial bone on the inside of the jaw.
The reason the image you found doesn't have those structures is because just as you said, it is NOT a polypterus. Not all splenial bones will be identical between different animals, creatues are more specialized than that.
If you read further into that study I already posted, and there is a lot of info in there, the only problem is that it is full of technical jargin that you need schooling for. I was able to manage by using dictionary.com lol
Here's the paper:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1262970/
Scroll down to figure 35 and 36.
Figure 35 shows the splenial bone (with the structures you keep refering to as teeth, but they're not). You'll notice that those structures vary in size and extend almost completely along the splenial bone, sure they are not used for chewing or to hole prety on the linguistic side of the jaw, plus those are too small.
Rather I believe they are an attachment point for the flesh itself.
Figure 36 shows the jaw (maxillary) without the splenial. Notice how those tooth-like structures are missing.
I hate beating a dead horse, just don't like being told I'm wrong when I'm not. lol