The book "A Thousand Guineas Plus" (Neilson and Warburton, 2013) has four similar pieces listed under the heading "Five Guineas" but they are all 31-31.5 mm diameter. All are dated 1762.
9760
O. GEORGIVS III DEI GRATIA. CHARLOTTA REGINA:, facing portraits of George and Charlotte
R. Pre-1787 shield and legend, no stops in legend, band of crown has 3 diamonds and 2 ovals.
Plain edge, die axis 0 degrees, 31 mm
9762
As last
Plain edge, die axis 135 degrees, 31.5 mm
9763
As last
Plain edge, die axis 180 degrees, 31 mm
9765
O. as last
R. similar but with stops in legend, band of crown has 3 diamonds and double pearls
Plain edge, die axis 180 degrees, 31.5 mm
All are given a value range of 6, which equates to £25-£45 (2012/13).
Were it not for the much greater diameter, yours would appear to be one of the first three, since there are no stops on the reverse.