My intention had been to update the Solr Reference Guide's Learning To Rank section w.r.t. External Feature Information i.e. https://cwiki.apache.org/confluence/display/solr/Learning+To+Rank#LearningToRank-ExternalFeatureInformation based on the https://lists.apache.org/thread.html/17547c1487067804077b1933ccde28d68841ec1694f6776c5682f28e@%3Csolr-user.lucene.apache.org%3E user mailing list thread.
In the process I noticed that configuring a feature such as this
and then not passing efi.preferredManufacturer=something is not rejected and instead a score of 1.0 is returned for the feature.
This seems unexpected i.e. a bug.