Hey Mike B,
The main noticeable difference is the fact that 55.2 had a tube that the turn signal indicator slid over. 56-59 had three little tabs that the turn signal slid over. See the difference next to the arrows between photo two and three.
Early 55.2
without the tab preventing the pods (right and left) from being interchangeable inside hexagon. Notice the round hole for the turn signal indicator.
http://i103.photobucket.com/albums/m127/59apache/Early_55.2_zpsnd1ydct9.jpg*quick note, the actual pods on both sides were also interchangeable. The hole for the oil sender and the temp sender were exactly the same sizes. I don't have a picture handy.
Later 55.2 or possibly early 56 with the tab preventing the pods (right and left) from being interchangeable inside hexagon. Notice the round hole for the turn signal indicator.
http://i103.photobucket.com/albums/m127/59apache/Later_55.2_zpspe6zr3gd.jpg56-59 with the tab same as late 55 inside hexagon. Notice the three punch out tabs where it was round in the previous years.
http://i103.photobucket.com/albums/m127/59apache/56_59_zpsldcivalk.jpgRoy I think you're right about 56 being the first year for the electric temp gauge.
Just a reminder paint on the backs of these things was a no-no. I took the shots before I removed the paint.
Some more cool geeky trivia:
In 55.2 and possibly early 56 the actual indicator color came from a film glued to a one inch paper tube like the cars had. The same one inch paper tube in 56-59 now had a little plastic cap instead of the plastic film. Early? 56 electrical temp gauges had three dots like in 55 but later on they got two dots and the line in the middle. It is amazing how a junk lot at a swap meet allowed so much learning. It has been a really cool journey so far.
***If anyone can correct this please do! I am no expert just interpreting the differences I see in these three clusters.
