Old Effigy vs Modified Effigy Halfcrowns, Shillings, and Sixpence of George V

In the image above, the left coin is an example of the Old Effigy type. The right one depicts a coin of the Modified Effigy type. The overlapping date in the series is 1926 and to the untrained eye, differentiating the two types can be difficult. Once you know what to look for, though, it is a fairly easy difference to spot.

Note the positioning of the initials B.M. on the next truncation. Unfortunately, on coins with wear, this is usually the first feature to disappear. The border is a telltale sign, though, and if you look at the beading or denticles, you can see that on the Modified Effigy coins, the border extends inwards much farther than on the Old Effigy types.