If I had to guess, you have a cep-packed handle at position 9-1 which overrides the override directory in a module. I don't use cep as a whole, so I don't truly know for sure. So you need to create or find an empty-model handle to use. You can either locate an unused slot in the cep longsword handles and use that slot, or edit your baseitems 2da longsword or cep-longsword entry to allow for greater number diversity. I havent looked, but I am guessing cep-longsword also only goes from 10 to 100.
To find an empty slot, without altering 2da's, just cycle through the available handle appearances and if one comes up as a brown sack, or if that number is omitted, then it should be empty and available. Then just edit the index number of the files (wswlz_b_091 and iwswlz_b_091) to fit that slot (wswlz_b_0X1). You might actually have better luck changing the color index and looking for a blank. Remember those go up to 9 also, so if your toolset shows a highest number being lesser than 9, then you can use any higher number than shown, up to and including 9, because you don't currently have a model for those colors. (wswlz_b_09X).
Don't read further if you don't want a headache...
This is exactly why I up all my number ranges to 10-250 so I get 675 (9 * (25 + 25 + 25)) unique weapons, minus three (total) for empty-model for each top bottom and mid. I know vanilla NWN's method gives you up to 140625 (9 * (25 * 25 * 25)) non-unique, but interchangeable weapons per baseitem line, but I prefer very specific looking weapons like you are using now. (does anybody know if that math is right, I certainly don't max mine out at all, even adding weapon effects right on the weapon models)
If you really want to get crazy (and I bet you don't right now), most, if not all, weapons will also allow you to use them as single-part items, in which case you can change your range from 1(0??) to 255 and have 255 (256??) unique items without having to bother with top mid and bottom plates. However, that may or may not screw with the toolset, so you might have to use a GFF editor at that point. I don't suggest it unless you have your own software and know what you are doing.