In older threads, from before we switched to this board, links to other threads are now broken because of the different naming convention. If you take one of those broken links and plug it into the wayback machine, you can still view the old thread and on the vB version of the board. And in that old thread, you can hover over the poster’s usernames and get the number.
The trick is finding a link to a thread you’ve posted in. For me, the easiest way was to a do a google search for my user name (which didn’t have an underscore, making it easier to find old threads), along with “this thread” (hoping for a link from one thread to another) and site:boards.straightdope.com
As an example, doing that got me this thread:
In that thread, I posted a link to this thread, as seen here by the wayback machine:
and from there you can hover over usernames to get the numbers.
Side note: interestingly, the old vB style links now work. They still show the old URL but it takes you to the current version of the page.
(ETA, then stick that number in your profile somewhere so you can find it when we do this again in a few years)