If I understand the codebase correctly, the _version.ts / _version.mjs file is supposed to ensure that the Workbox version is included in each of the build JS files:
try{self['workbox:recipes:7.4.0']&&_()}catch(e){}
However, this does not appear to be updated properly for all packages; for example, workbox-routing 7.4.0 still contains self['workbox:routing:7.3.0'] && _().