It's MSC 5.10, MASM 5.10, LINK 3.65, NMAKE 1.00.05, and some other stuff.

Comes down to around 2 MB after cleaning up a bit. 1.4 MB if only keeping support for one memory model. Runs fine in DOSBox, so this is an easy way to make a project self-contained, building from within the target platform with no external dependencies.

Thanks for the overview.

I did not had an environment to try it out, great that I didn't land that far off just from my memories. :)