Virtus Walkthrough 2.5 Sources
This is the only version of the source code for Virtus Walkthough that I am aware of. It was the last version before the product was restructured to create ConceptCAD. This is made available fully open source with the Apache 2 license.
https://www.macintoshrepository.org/38511-virtus-walkthrough-1-x
https://www.macintoshrepository.org/295-virtus-walkthrough-pro-2-0
https://wiki.preterhuman.net/Virtus_Walkthrough_Pro