By default, when the upstream argument is specified for the exwsAllocate step, the most recent stable upstream build number is used when allocating the workspace. This behavior can be changed by passing a specific build number to the plugin.
For this, add an extra parameter to the exwsAllocate step, that is buildNumber.

e.g: def externalWorksace = exwsAllocate upstream: 'upstream-job', buildNumber: '12'

If this parameter is provided, the step will allocate the workspace used by the specified build number, within the upstream job.
If the provided build number doesn't not exist in the upstream job, or if that build is not stable, an exception is thrown.

For additional details, see https://github.com/martinda/external-workspace-manager/blob/master/README.md#changing-the-upstream-build-number