The 1852 "Small Head" Wass, Molitor & Co. $10 is one of the more unusual Pioneer gold coins. The obverse shows a raised circle around the 2 of the date. The circle was caused by a steel plug with an incuse 2 that was placed into a hole drilled into the obverse die. This was done to either correct or redate the die. The plug and the die face were not level, hence the "circle."
The "Small Head" designation is a traditional name that fails to adequately distinguish between the so-called "Small Head" and "Large Head" varieties. In actuality, the two heads are close in size, but differ in shape. Kagin, in consultation with Breen, came up with new designations, but without his book, even these are difficult to use. Kagin calls this the "Long Neck, Large Close Date" variety. Better to simply recognize it as Kagin 3.
Kagin called this an R-7 (7-9 known) in 1981. In 2014, approximately 15-20 examples are known. The finest example is the PCGS AU55 that sold in 2009 for $23,000; however, the record price belongs to an NGC AU50 that sold for $25,850 in 2014.