[_ck_]

They are trying to link to the first reply.

It far easier to inject an anchor at the very bottom of the first post and link to that instead.

It could be done via a plugin that appends post_text and checks the $bb_post->post_position to see if it’s equal to 1

If it’s equal to 1 then append <a name="first-reply"></a>

Then your first reply is always at http /topic-link-blah-blah#first-reply

Note that my unread-posts plugin already solves the problem of linking to the first unread reply per user.