This is Bill Joy's ex(1) from 1977, included with the 1BSD release. The
source code was extracted from The Unix Heritage Society's Unix Tree.
The original code will not be tampered with and everyone can look at it first hand as it was. I'll attempt to bring it up to modern times and compile it on a modern machine, but that'll be done in different branches.
To the best of my knowledge the source code at The Unix Heritage Society's Unix Tree does not come with a specific license. I assume the license is inherited from the original source code distribution. BSD code was written under the University of California copyright, however this work does not come with the famous "The Regents of the University of California" label. The copyright here is Bill Joy's.
With the absence of licensing it is unclear whether we are allowed to modify and redistribute the code. However, it was given to The Unix Heritage Society by Kirk McKusick and Keith Bostic, both early BSD developers, and I assume with the intention that it would be freely available to the world at large as a piece of the computer industry history.