This is an interesting tombstone inscription I have seen a few times, the symbol of the Freemason order of Knights Templar. The inscription features a Patee cross with the Latin phrase In Hoc Signo Vinces. There is usually a Christian cross and crown in the center, although the cross and crown is sometimes used alone. The Latin phrase dates back to 312 A.D. and the Roman Emperor Constantine and is translated as In This Sign Thou Shalt Conquer.
Wikipedia gives the full name of the Knights Templar order as The United Religious, Military and Masonic Orders of the Temple and of St. John of Jerusalem, Palestine, Rhodes, and Malta. The order is open to Freemasons who profess a belief in Christianity.
A little different version of the Knights Templar symbol:
The original Knights Templar order (Order of the Poor Knights of the Temple of Jerusalem) was founded about 1118 A.D. to protect Christian pilgrims traveling to the Holy Land after the First Crusade (1096-1099). They were known across Europe as an elite fighting force with a strict code of conduct. They swore oaths of chastity, poverty, and obedience and eventually gained a great amount of financial and military power.
Pope Clement V officially dissolved the Knights Templar in 1312. Since the 1700s the Knights Templars have been revived and imitated. Today, in addition to the Masonic order of Knights Templar, some non-Masonic groups have Knights Templar orders.