I'm not a coin guy, but if I'm reading correctly the half penny and shilling may not have been made in the previously guessed years (see link and let me know if I'm wrong). In trying to match up the details I'd guess it's what the link is calling a type 1, made from 1770-1775. I'd guess at 1773 from what I see, but could certainly be wrong. British Coin Gallery - George III
And I'd gues its a half penny, but we need to know the size to confirm that (or at least I would).