ah aha! I think the variable isn't even used anymore:

$ git grep master_agent_version
CHANGELOG.md:- Base master_agent_version on pe_compiling_server_aio_build().
HISTORY.md:- Base master_agent_version on pe_compiling_server_aio_build().
acceptance/helpers.rb:      master_agent_version = fact_on(master, 'aio_agent_version')
acceptance/helpers.rb:      unless master_agent_version
acceptance/helpers.rb:          initial_package_version_or_collection ||= master_agent_version
manifests/params.pp:      $master_agent_version = pe_compiling_server_aio_build()
manifests/params.pp:      $master_agent_version = undef
manifests/params.pp:    $master_agent_version = undef