You need to change the IDString value to just "SP" then add "get ID short" so, like this:
get ID short
However, there's another problem. Some of the SEP files combines Sony ADPCM audio data and the AT3 file itself. And those files have a very confusing offset calculation. That's why the script fails once it reaches those kind of files. I'm currently still working on it to fix the script.
Now we're rolling! I've spent a few hours extracting from oss_s01_b.sep down to oss_s19_01tel.sep. It's got the cutscene dialogue that I want. It appears you may be right though, I think some files are being missed after cataloguing them by scene. It's not a lot. I've looked at about half the cutscenes of the game (they are organized chronologically) and I'm going to guess the rate of missing dialogue is about 20%. Some cutscenes just don't have an associated sep file while some other scenes are just missing a handful of dialogue pieces. If they have anything in common, it seems be more action-y sequences. Where dialogue, music, and sounds of battle are ramping up. In previous MGS games, some story-prominent codec calls would have the music baked in to the dialogue, so maybe this is Portable Ops' version of that.
Of course there is a possibility that some of the files I've yet to look at are numbered out of order and contain what is missing. Or maybe the script works on the files with "bgm" or "se" in their name and the missing content is contained somewhere within them. I'm crossing my fingers as I keep looking.
As for why the script is failing, I wish I had an answer. I can only offer a potential clue. I seen .sep files referenced among PS1 games as containers for "seq" files, including konami games like MGS1 and the PSX port of Snatcher. Maybe my assumption that these are meant to be Sony Atrac3 format was off base? I wouldn't know how to tell what a file is supposed to be just by looking at it, but this webpage talks about headers: https://loveemu.hatenablog.com/entry/20 ... SEQ_Format