British 1889 Half Penny 9 Over 8 ?

Correct 1889/8 but your positioning is slightly off here is where the 8 sits marked with arrows and below a mock up of what it would of looked like and your original at the bottom very nice find

Known as good housekeeping and the shortage of metals to strike the coins it was common practice to remove the last digit from the die and replace it with the new year digit in this case the 1888 die was or should of removed all trace of the 8 before placing the 9. Obviously someone in quality control would of picked up on this and the die would of been either repunched after removing any trace of the 8 . Or the die was coming to the end of its life literally So they unsuccessfully left some of the 8 in place before altering the date probably 10-50 strikes before the die broke into pieces and so a rarity occurs as the brand new die would just have 1889 from the get go.

Some dates are in fact almost all over dates like the jubilee shilling 1888 nearly all were struck over 1887 . There are true 1888 shillings but are very hard to find