Publication 52, Hazardous, Restricted, and Perishable Mail > Appendix C > USPS Packaging Instruction 1B
Safety fuses consist of a core of black powder overspun with yarns, waterproofing compounds, and/or tapes. A safety fuse assigned identification number UN0105 and classed as a Division 1.4S explosive may be mailed only as permitted in 341.22b and when properly packaged as follows.
Prior written permission must be obtained from:
Manager, Product Classification USPS Headquarters 475 L’Enfant Plaza SW, Room 4446Washington, DC 20260
Note: Full responsibility rests with the mailer to comply with Bureau of Alcohol, Tobacco, Firearms, and Explosives (BATFE) regulations before mailing. A legible photocopy of the Product Classification Manager’s approval letter must be presented by the mailer to the Postal Service acceptance clerk at the time of mailing.