The appearance of the console changed over the years just as the operating system and the printed circuit board were subject to constant adjustments. Unfortunately changes have not been recorded and could not be identified even by the week and place of console assembly.
This page shows the evolution of the right hand side of the console and answers the question "what was the solid state badge for".
The left hand image is of an original NTSC TI-99/4, which had a video and audio feed to an NTSC monitor. For Europe TI chose to have a different and unique video output requiring a special modulator. This they had designed and manufactured in a European country other than Britain, unfortunately Britain used a later and (then) none-compatible PAL TV system to the rest of Europe. Later developments in TV production were better able to handle the different standards.
The result was a European modulator which had no sound on a UK tv, and a quick fix was to insert a small speaker behind the grill behind the module slot and a slider volume control at the front, which is visible in the second image. There was also a small audio amplifier.
By the time the TI-99/4a was released the British audio problem was ALMOST solved, and UK users endured either a slightly off-tune image or a noisy sound channel but the internal amplifier went and the badge filled the slider slot.
The same console case was used for a while but ultimately the front of the console became quite flat (image 4).
Towards the end with the beige console the mains power switch was moved to be in front of the module slot, from the edge of the console.