An anchor was a fairly common symbol at the time--I've seen them too. I think in this case it meant an untimely end to life. A similar symbol was a gravestone craved like a tree stump, which interprets as "cut down in the prime of life." Where's what I found about the anchor:

"The anchor was regarded in ancient times as a symbol of safety and was adopted by Christians as a symbol of hope and steadfastness.

The anchor also represents the anchoring influence of Christ. Some say it was used as a sort of disguised cross. The anchor also serves as a symbol for seamanship and may mark the grave of a seaman, or be used as a tribute to St. Nicholas, patron saint of seamen. And anchor with a broken chain symbolizes the cessation of life."

Kent in SD