1820 Half Dollars offer the collector a dizzzying array of possibilities: Large Date, Small Date, Overdate, Square Base 2, Curl Base 2, Knob 2, Plain 2, No serifs on E, and combinations of the preceding. In this case, we have an 1820 Half Dollar with a Square Base 2, a Large Date, and a Knob 2. Jse the image above to determine the proper variety.
According to the PCGS Population Report (as of September 2011), the 1820 Square Base 2, Large Date, Knob 2 is less common than the Curl Base 2, Small Date and the Square Base 2, Large Date, No Knob, but not as scarce as the 1820/19 Square Base 2 and the 1820/19 Curl Base 2.
Mint State examples of the 1820 Square Base 2, Large Date, Knob 2 Half Dollar are very scarce and the finest example certified by PCGS is a single MS-66.