The entries in /usr/local/tce.installed serve both as the indicator of the extension being installed as well as the startup script if the file is executable. Killing two birds with one stone rather than having a seperate file serve as the startup script.
If /usr/local/tce.installed/bash is present in the system, the appbrowser or tce-load will state the exetnsion is already installed instead of fetching/installing it. During the installing of an bash (if /usr/local/tce.installed/bash does not already exist), /usr/local/tce.installed/bash will be ran as a script if the file exists in the extension as an executable script.