I believe that the older, large 4 slot ESL detectors came in 3 distinct generations and I’ll explain below.
On the left is my ESL 509C which is what I believe to be the oldest of the 3 (could very well be the newest). The visual differences that can be noted is the large, central ESL logo, no text below the LED, and no bouding box around the heat sensor and LED. These also use the plug-in style base (see image below) and have text on the side that reads:
“TO REMOVE ↑ PRY HERE
AND ON OPPOSITE SIDE”
This is how you remove the base, and this is what I’ll call the 1st Generation.
In the middle is my ESL 523D. Ignoring the fact it has a transformer on the back because it’s 120VAC, it does have some changes. First, the aforementioned bounding box around the heat sensor and LED is present. There’s also now text below the LED that reads “ON/ALARM”, as well as a smaller ESL logo above the heat sensor. This still uses the plug-in base, and feature the removal text. This is what I’ll call the 2nd Generation
On the right is my Simplex 4259-36 (I forget the ESL model). Visually, it looks almost exactly like the 523D, but it uses a twist-on type of base. Unlike the other two, it also uses screw terminals instead of wire leads (and is also 2-Wire). Due to the new base, there is no longer any removal text. Finally, this is what I’ll call the 3rd generation.
Plug-in base on the left (from my 523D), twist-on base on the right
Feel free to add or correct any information above. I don’t know how common of knowledge this is, but I thought I’d share my observations.
No, I can’t figure out if there is any internal changes as you cannot open these heads as far as I’m aware
The edits have changed some wording and added information