Been meaning to write a reply to this -- here's what I have on the case...
A reference collage of the front:
Back of the case:
The interior panels seems to stay the same through the show, but the tools change.
2x21 "Peak Performance"
Has the modular tools, the Sulu tool, engineering PADD and some other misc items.
4x18 "Identity Crisis"
Looks mostly empty...
From later on in the run of TNG as the red PADD is in its final form (Season 5-6?)
Here's a photo of the PADD-side insert, posted by HMS
And another with it on a cutting mat for scale reference
The exterior plant-on, again from HMS. Previously Mike stated that the master for this casting came from a surplus store, IIRC. Only one is used on the case and it goes on the side with the latches (the PADD side). Looks to be painted same grey as the case, and at times had chrome tape around its perimeter.
The text on the outside of the case reads "ENGINEERING FIELD KIT" in one of the Compacta fonts.
4x19 "The Nth Degree"