Just to have this for reference. If you have some older m0n0wall machines (1.34 and earlier), the 1.34a version that use to be the stepping point to 1.8.1 m0n0wall (and thus upgrading to SmallWall) has disappeared off the m0n0wall site and mirror sites (maybe years ago, sorry if it is somewhere else now) But there is a way to upgrade from the earlier versions to 1.8.1 without needing the 1.34a version.
Open up your 1.34 version, use your browser to create (2) tabs to the same configuration page.
On the first page, go to the System / Firmware section (system_firmware.php) and Enable the firmware upgrade mode. Go ahead and select the 1.8.1 version image to prepare for the upload.
On the second tab, go the exec commands page (exec.php) and execute this command:
umount /ftmp && /sbin/mdmfs -s 16m md1 /ftmp
After that is complete, switch back to your first tab at (system_firmware.php) and do the upload/upgrade. Provided your system has the minimum system specs to run 1.8.1, you should get a successful upgrade without needing 1.34a to do it. From that point, you can get to SmallWall. I hope this helps anyone else that took some time to finally upgrade some systems (years later, haha)
Ok, it wasn't just me. I spent an hour looking for that image file, checking archives, mirror sites, etc. I couldn't find it on the m0n0wall site anymore but everything else is still in the download tree on the old m0n0wall site, not sure why it disappeared.