Good old-fashioned PETN, I don't doubt. Or an analog. Nitrocellulose requires good confinement, hard to do reliably in a pager. relay.nostr.net

I was just riffing off the word "cotton" which ben got from Snowden saying "cotton on to". We are off in the weeds. But I'm going to guess it was the more expensive and difficult to find mercury fulminate (with some potassium chlorate) undetectable by explosive detection systems looking for nitrate...