At work (National Center for Atmospheric Research), we commonly stored tape contents in the Cray COS blocked data set format. It is pretty reasonable as it contains record and block control words that allow you to seek both forward and backward within the dataset, and to retain tape marks as end of file or end of dataset marks. There were many tapes containing historical weather observations ingested and stored on the Mass Storage System (MSS) in this format. While the MSS has been replaced by an HPSS archive, most of the 2 PB of data were migrated into it, so undoubtedly there are still COS blocked data set tape images in the archive. If anyone has data stored in this format, I do have some C code that will read and write it.