The Starrett plumb bobs, style 87 series were mercury filled, older ones were round top, newer ones have a hex top so the don't roll away when laid down and a hardened steel point that is black instead of chrome plated like the rest of the body. The 177 series were similar looking to the 87's but were solid steel, these didn't weigh as much as the 87 series. I think there were 4 different sizes in each series.