This product is related to magnetic "developer", a bottle of fine iron filings used to see data on magnetic tape.
If you had a reel of tape that was damaged somewhere in the middle, you could use the developer to see record boundaries and splice out the damaged record, which would enable most of the data on the tape to be saved. This was possible because reel tapes were longitudinally recorded in a single stream, with 9 tracks in parallel each representing a bit in a byte. There was a gap between each record where splices could be made without disrupting the data.
When tape decks started to switch to helical and then serpentine recording (approx. mid-1980s), it stopped being possible to splice tapes while preserving the data.