That’s the topic.id
That’s the post.id
The title is part of the topic, raw is part of the post. If you look closely in the post, you’ll see a topic_id field that contains the number that you’re calling the private message ID.
You can have a look here: There are 2 IDs when I try to get API, in case of lack documentation (that’s this topic, but it’s the same as PM) and see:
So that’s the post_id is what you’ll use to POST changes to raw. The topic_id is where you change the title.