Without downloading an extension, you can simply hit the "quote" button on any post in any other open thread (such as this one), and then on the new page that loads change the post number in the URL (quote=XXXXXXXX) from the post number you hit "quote" on to the post number you actually want to quote (in this case, 56601548), and reload the URL. You can then copy the bits of the BB code that you need. If you are going to actually quote the post (rather than just use the BB code), then you also need to change the topic number in the reply, or it will still try to link back to the thread you are in instead of the original locked thread.
Find the post you want to quote, and click on its title to reload the page to that specific post. Then copy the message number (last 8 digits) from the URL.
Then, in the thread you want to reply in, click on "quote" on any post. In the URL bar on the new page, paste over the 8 digits after "quote=" with your new 8 digits. Press enter.
To ensure the quote links properly, you then need to take the 7 digits after "topic=" from the original post (as in step 1), and paste them over the digits after "link=topic=" in the body of your reply.
Having said all that, the BPIP extension works perfectly for this and gives you a bunch of other great stuff too.