Join Date: Jan 2007
As stated above, this type of top was a safety device, as occasionally 'normal_flasks would blow up on charging a barrel containing a spark.
Some tops looked like the regular, but were warranted fireproof.
This type of top appears to have been used more in France than the UK,.
Anyone with the Powder Flask Book will see many variations.
Sorry though, I don't own the book!
It's a nice one to have, and much scarcer than the usual types.