While I don’t specialize in coins from Guernsey, I do specialize in overdates. So when I spotted a Guernsey 1 Double that is
widely attributed as “1868/30”, I decided to do a careful analysis.
The Guernsey 1 Double coin was first struck in 1830, next followed by 1868--both at the London Royal Mint. While I understand the apparent logic behind the “1868/30”, I’m skeptical of this attribution. Firstly, a date spread of
38 years is
uncommon for overdates. Obviously, it requires the mint to store a die for many years without rust damage, and then fit the older die to a press which may have easily changed over those 38 years. Secondly, the Royal Mint changed planchet compositions in 1860 from copper to bronze. Striking coins with a harder metal like bronze on faster presses than used in 1830, requires tougher dies. And no doubt, die steel alloys improved over this time to accommodate the greater pressures needed for bronze coins. So an 1830 die on a press in 1868
may have not worked very well.
The obverse and reverse of the 1868 Double—a small, simple coin worth 1/80 of a French Franc. While this coin was struck in London, French coins were also used in Guernsey at this time.

Below--a detail of the date, showing the “1868/30”, with corners of a digit clearly protruding from the top of the 6. Even at this magnification, it can be seen that
the mark on the 6 is blunter on the left, coming to a sharper point at the right. This is an important detail which I will discuss below.

To discuss the plausibility of an 1868/30, I now compare the date on the 1868 to the 1830, scaled equally. The white bracket shows the width of the mark in question, compared to the top stroke of the 3 from 1830. Given the size and shape differences on the 3, I don’t think a 6/3 adequately explains what I see. However, by comparing the mark to the top of a 5 taken from an 1858 Guernsey coin, I see a significantly closer match. Notice how the shape of the top of the 5 more closely resembles the mark under the 6—ie blunt on the left and coming to a sharper point on the right? Based on that evidence, I’m more persuaded of a 6/5 in this case. Indeed, markers like this are seen on 6/5 overdates of this period—such as this coin from Sweden--second picture.

The details on the Guernsey coin show similarities to this 6/5--a blunter mark on the left, a sharper point on the right:

Finally, there are stark differences in the style of "1 Double" on the 1868 die which clearly show it's
not a re-dated 1830. The lettering on this 1868 matches other 1868 reverse dies.

While the 1 Double was
not struck in the 1850s, I
suspect this particular overdate shows either a proposed die run during that time, or possibly just a misdated die from 1868 that was fixed and used. I would suspect 1868/5X, with the last digit a 0 or perhaps an 8 that was shifted up. A date spread of 10-18 years is
more plausible too.