ID of NATO code "RPB" on AN-M14 TH incendiary grenades

Is anybody able to ID the NATO factory code “RPB” used in AN-M14 TH incendiary grenades?

This is what we used to call a thermite grenade, impressive, will destroy anything/everything it is put upon… I remember them fondly!

The AN-M14 has ‘Thermate’, an improved form of Thermite, burns for 40 seconds. The latest version has 26.5 ounces of Thermate (TH3) mixture.

The only reference to ‘RPB’ I found this in an International Air Transport Association pamphlet:
“Class/Div IMP Code Description
6.1 RPB Toxic substances”
Class 6 is toxic, 6.1 adds a SKULL image