Here is a gorgeous 1827 Square Base 2 (for the 2 in the date) Capped Bust Half in PCGS AU50.  There are 49, yup, 49 die marriages of the 1827 Capped Bust Half and 44 of these are the Square Base 2 obverse from various dies.  This one, after spending over an hour perusing all guides, we are certain is O-142, unless we missed a key marker.  
 
In either case, wouldn't it be fun to collect all 49 die marriages?  Some folks in the Bust Half Nut Club surely have, and there are some R6 and R7 varieties in this group magnifying the challenge.  What would you start with to build that set?  We would start with this one.  Eye appeal bursting out of the seams as shown, the classic old album toning progression from aqua blue on the edges to iridescent grey in the centers on each side, and silky smooth cartwheel under the light, what is not to love?  Or you could just buy this one for whatever set you are creating, it has that special look that so very few silver coins from this era have.  Just the type of coin we love to offer but so difficult to find.
 
This coin is approved by CAC.
 
PCGS Cert. #45745594