The V did change to U but was back to V in 47. It's the younger bust, only dates 1729 to 1739 so unless the last digit is 9 there's no question about the date.
1757 does exist, but are counterfeits and quite rare, and to find one metal detecting would be incredibly rare. For these all the rules go out the window.
To the naked eye, it looks like a 5. But it certainly could be something else. I agree that it looks like the younger bust version.
Gary