I have seen a fair few of these and have a couple of my own, so not particularly rare even though not in Gouby's book. The die pair is 6 + G. [Freeman's notations]
The reason why the H is displaced is because the 7 has been entered closer to the 8 than on other reverses (the master die/matrix had just 18_ _ on it and working dies had the last two digits (and mintmark) hand entered, explaining the great variety of date widths and anomalies seen) and keeping the H central to these digits means it just ends up over a gap. Not to be confused with the very rare reverse I with wider, low date digits and H over gap.