This is the first milestone: A full release of 2.50, in stable form, 2.50.0 FINAL (Build 1606, internal ID 24).
This build was used for 7 months, with no updates, or changes.
The ZScript additions in this build are:
Game->GetFFCScript()
Game->DMapMap[]
From here through 2.50.3RC1, there were no major changes to the ZScript API. It remained static, through to 2.53.x.
This milestone build was incredibly popular, and is still in use today, despite numerous update releases.