Fun fact, GW-BASIC was a descendant of the original Altair BASIC. The "Translation created 10-Feb-83" headers on each source file refer to tooling Microsoft had that automatically translated the 8080 assembly to 8086 (it shouldn't be taken as a build date since they were manually modified after that point). Besides GW-BASIC, source code for the 6502 and 6809 rewrites of Microsoft BASIC were available up to this point (see https://www.pagetable.com/?p=774 and https://github.com/davidlinsley/DragonBasic) but I believe this is the first public release of the original 8080 BASIC code.