Set regression to "Yes", because I was able to open these legacy prefabs on earlier Improved Prefabs builds.
Also crashes on the latest trunk while instantiating (some of the Improved Prefabs PRs already landed, but not a main one).
Version 2018.3.0a6 (3e681eca9b01) Sat, 14 Jul 2018 23:14:21 GMT Branch: trunk

Steps:
1. Open attached project
2. Go to Assets -> Prefabs -> Props
3. Open "Bomb.prefab" or "rocket.prefab"
=> Editor crashes

Also, there are warnings like
"Failed to convert 2511236001 to a signed 32 bit int in file 'Assets/Prefabs/Characters/enemy2.prefab' (168:20)"
For most of the prefabs while you are trying to open them.