RocksDB is missing CassandraPartitionMetaMergeOperator source file for Rocksandra

We were attempting to rebuild the RocksDB distribution for Rocksandra. The Rocksandra code uses/refers to the CassaandraPartitionMetaMergeOperator class but this class file is not part of the RocksDB distribution. Inspecting the JAR file for RocksDB for Rocksandra shows a corresponding class file. So at some point someone built the JAR file from some source but we cannot find that source file anywhere.