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Neville
The rope coiled around the middle of the item rang a bell. In the 1870's -1880's ML mortars could be used for firing incendiary shells. The hollow shell was filled with some inflammable substance and the outer was wound with tarred rope. The tarred rope would keep burning after the initial flare and hopefully set foliage, trees, buildings and the like alight. The following is a bit off-topic - but the German Zeppelin incendiary bombs of WW1 were rope bound for that very purpose.
Tomorrow I will have a dig further to see if a definitive answer can be given.
Regards
IL.

Neville
Over the weekend I had a trawl back through British Ammunition Treatises (the earliest was 1877) and no such projectile was seen or mentioned. I don't have any reference to early Continental Artillery treatises - apart from what little is on-line. So, I would have to fall back on my original comment.
Best regards
IL.