The number in the URL is not the ID, that’s the post number, which is relative to the topic ID.
If you want to get the post ID you can retrieve it from the topic json as post_stream.posts[].id
So the posts in this topic have
| topic ID | post_number | ID |
|---|---|---|
| 255455 | 1 | 1241013 |
| 255455 | 2 | 1241026 |
| 255455 | 3 | 1241056 |
and your most recent post in this topic is https://meta.discourse.org/posts/1241056.json