|Damn, forgot the scale. The head is about 40mm diameter|
Anyway, this one isn't from a boat. I found it whilst on a Boxing Day walk, and if, as I am pretty certain is the case, it fell directly from above onto the towpath, it is from the Wicker Viaduct (span 41), properly called the Sheffield Victoria Viaduct, and better known for the bulk of its construction, the Wicker Arches.
Constructed in 1848, the viaduct last carried passenger traffic in 1996 (and last carried mainline traffic, to the now defunct Victoria Station, in 1970) but still carries local freight. (info. source here)
Do you think I should report that rivets are dropping out of it?