A well known variety. See screenshot from Michael Gouby's website below. Typically a master die in the Victorian period had just the 18.. pre-entered on it, and from that working dies were made that then had the last 2 digits hand entered. (This is behind all the date width varieties for example on Victorian bun pennies).
For 1857 shillings, one working die had the last two digits entered with numeral punches from the wrong font thus causing the example above.