If I can find a copy of the TM SGNL app, I can not only request the source code, I could also try reverse engineering the app itself to see exactly how it differs from the official Signal app... The Enterprise Version and Developer Version buttons link to docs explaining how to distribute the iOS app using either the Apple Developer Enterprise Program (used by big businesses to distribute custom or proprietary apps to their staff) or the Apple Developer Program (used by anyone who develops Mac or iOS apps – I have one of these accounts). TeleMessage distributes the TM SGNL app directly in the App Store, but as an unlisted app.