Yes, along with 1754 copper the 1758 shillings continued to be struck into the reign of George III. Note that Northumberlands and the 1787 BoE specials apart, 1758 was the last regular date for shilling issues until 1816, which is an enormous stretch. One can only speculate that they were struck even after the Northumberlands of 1763.
And there were only 2,000 Northumberlands I believe, which wouldn't make much of a dent in the demand for shillings and most probably didn't circulate ...
I remember you saying so after the grading results came through (a few years ago now!)
It seems to me that it is very hard to find them fairly nicely struck on the centre of the rev, most seem to have that slightly mushy look
No recorded mintage, but very common as they were struck over a number of years. Yours has graffiti in the angles that has been partially removed and it looks to have suffered from a bit of haymarking. I suggest it was dug up at some point looking at the surfaces.
I couldn't find many examples of the 3d but I'm still new to finding exactly where to look, 15k would suprise me (cheap at that price) from my limited knowledge of gold - mostly because I can't afford it so I try not to cause myself pain by looking