I've always assumed that the numbers are assigned without regard to original board, which is what makes the search by number so hit-or-miss. If the message number is on geek and your message number is a post on off-topic, then you'll get a miss. It's not the easiest way of digging, but you do get some gems now and again.
What would be great is to be able to make bigger leaps with the earlier button. I can search that way and find some things I recall, but it is time consuming. So if it could generate a TOC as if I'd just hit the earlier button 10 times, or a hundred times.