I wouldn't call it an error, rather just a variety.
It appears that a '3' is under the 4 in the date from your pics, to me this is a repunched date(rpd).
When they finished minting 1739 issues the reverse die with the date was still usable so it was saved and the date was repunched for 1740 coins of the same denomination.
It was a common practice to save usable dies until they were absolutely unusable.
Other examples of similar practices are repunched mintmarks, retooled devices, and weak strikes caused by overpolished dies..etc
Although rpd's are common they do add value and interest to some coins , in this case there probably isnt much premium on this .
Really nice find, great shape too!