This is a very interesting variety that pairs an 1840 Seated Liberty obverse with a reverse die that was used to strike a Capped Bust Half Dollar! Though this is quite unusual in itself, comparison with 1839 Capped Bust Half Dollars revealed that the reverse came not from a Philadelphia Mint coin, but from one minted at New Orleans! This can be explained by the fact that, in 1838 and 1839, the "O" mintmark was placed on the the obverse of the Half Dollar, not the reverse. So, it seems that the New Orleans Mint was practicing a little economy back in 1840, when it reused an old die from 1839 to strike 1840 Half Dollars. This is a perfectly reasonable thing to do and was not without precedent...it is just that dated obverses are a lot easier to tell than a reverse without a mintmark. Thankfully, research into die varieties on American coins has progressed far enough to make such a determination.