Square cross-secton cascabel on these gunades. Why?

[cannonmn]

Near the end of this article are pix of wreck-recovered gunades with square-cross-section cascabels.   Why?  Did something fit on them that's now missing?  Could some novel gun construction technique have featured a separate piece screwed-in that was made so it could be turned with a wrench,?
I saw a very similar piece at an intersection somewhere in New York long ago, with the same type cascabel and have wondered about it ever since.
 http://www.hmssirius.com.au/recovery/artefacts

[steelcharge]

Not sure, but I think somekind of a handle or hook was attached to that part. I've seen some drawings of originals with the hook/handle attached, but don't know what was it's use.

Here's a video of a 9-pdr gunade/carronade, raised from the "Stormwreck" couple of years ago. It still has, I think half of the original "hook" present.
http://www.youtube.com/watch?feature=player_embedded&v=r_HdsZAA52U

EDIT: This is the drawing I was thinking about (first one on page):
http://men-o-war.blogspot.fi/2011/04/weaponry.html

[Bob Smith]

Those are actually very early carronades, before they took the took off the trunnions. They had a cast-iron breechring which has broken. Brian Lavery has written about their development, back in the late 1980s or early 1990s.
 
Bob Smith