I have also seen deuces with an "add on" tach under the dash. It is the same as the in dash tach, mounted almost directly under the gauge panel. It mounts with a very simple bracket, similar to what racers used in the Sixties to mount the big, distributor take-off, cable driven race tachs (think Moroso).
I surfed the web this morning to try and find a pic, but came up blank. It's been a while since I saw one. A few years ago, Oshkosh Equipment had a couple of Deuces so equipped.
I do not think gassers had tachs.