The project was never licensed, it only contained third party licenses.
I determined to use the Source SDK 2013 license for this, as the majority of the business logic running this product is based on Valve's (the license has zero restrictions in the scope of our goal with this project).
The licenses has to be included with any depots from now on in the folder 'legal' placed in the root of the project folder (the location of r5apex_ds.exe).
With any new additions of third party code, the 'thirdpartylegalnotices.txt' file has to be updated accordingly.