Having recited the specifications that are common for all the back-plates, now let's describe the specifications that vary between the different back-plates.  Feauture that differentiate the various Heuer back-plate include the following:
<UL>
<LI>base material: brass or aluminum</LI>
<LI>coating material: chromium, profectus or black anodized</LI>
<LI>number of holes to attach plate to the dashboard: two, three or four</LI>
<LI>cut-out for crown or none
<LI>marking on the back: none, number, Ed. Heuer & Co., Heuer-Leonidas, S.A. or Sears, Roebuck & Co.</LI>
<LI>finish of the back: smoothe or matte</LI>
<LI>overall thickness of the plate: 3.4mm or 3.8mm</LI>
</UL>
<p>
Now let's work through the chronology of the back-plates with an eye on these specifications: <p>
<UL>
<LI>Early Back-Plates: <UL><LI>chromium coated brass;</LI> <LI>three holes for the Autavia and Auto-Rallye;</LI> <LI>cut-out for crown;</LI> <LI>smoothe back either has no marking (earlier) or a number (later);</LI> <LI>3.8 mm thick</LI></UL><p>
<LI>Ed. Heuer & Co.: <UL><LI>brass plate is coated with either chromium (earlier) or profectus (later) [see the photo above or <a href= "/Guide/_Accessories/5.Back-Plates/Ed._Heuer/?show=ChrProf6BackBst.JPG">click here</a> to see the difference in the color of these finishes];</LI> <LI>two, three or four holes to attach plate to the dashboard;</LI> <LI>back has smoothe finish (earlier) or brushed finish (later) and is marked Ed. Heuer & Co. Swiss;</LI> <LI>3.8 mm thick</LI></UL></LI>

</UL>