They were packed with scrap iron, pieces of barbed wire, used cartridge cases, stones or any objects that would act as schrapnell.
They would then be filled with explosive and a simple bickford fuze to ignite it.
This model has a correct period label and even has tufts of gun cotton sticking out the seams of the lid.
If you want one you can find them at Geoff's great web site
www.tommyspackfillers.com
theres a link on my home page.