Nice coin, Matthew. And valuable, too.
This is a 1/3 guinea of George III. The crown on the reverse side is common to all dates between 1797 and 1813, but the inscription changed from MAG BRI FR ET HIB REX to BRITANNIARUM REX FIDEI DEFENSOR in 1804. It does not matter which inscription you have, they are all valuable, as shown below.
1/3 GUINEA: 0.0821 troy ounces gold
Note: if you have a half guinea or one guinea coin, it looks different and carries different values. See this page [PRESS HERE] for half guineas and guineas. This page applies only to one-third guineas as shown in the picture.
worn: $150 US dollars approximate catalog value
average circulated: $250
well preserved: $550
fully uncirculated: $1300
1/3 guineas dated 1811 are especially valuable, multiply these values by four
1/3 guineas dated 1813 are somewhat more valuable, multiply these values by two
The coin in our picture comes from Fritz Rudolph Kunker in Osnabruck, Germany where it sold for 200 euros (about $220 US dollars) during a 2015 auction. CoinQuest thanks Kunker for use of their coin photo. It's a nice one!