The Seahorses can be incredibly difficult to identify, but the NSDA guidelines are excellent and should be followed. Assuming horizontal lines, and a vertical measurement of close to 22.75, then the ID must be 179-181. The dot itself is sometimes strong, and sometimes weak or non-existent. Here are some closeups of my 179, 180 and 181 with the dot in the 179 very strong, 180 weaker, and 181 very weak, but there.



I once had a problem identifying my copy of 181, confusing it with the De La Rue 175c. Here are both of the stamps, with 181 a mint copy, and 175c used. Note that I added a red arrow to point out the larger top teeth seemingly seen in both the 181 and 175c.


The problem is, though, that the 181 vertical measurement is 22.75, as it should be, while the 175c vertical measurement is 22.0, as it should be. So, it just goes to show you that you need to follow the directions to positively identify your stamps. No matter how much I may desire that my 181 is showing the larger top teeth characteristic of a mint 175c, it is still only a 181 based on the 22.75 vertical measurement. Darn, a 181 mint is catalogued at about $425, while a 175c mint would be $3150.
Note that my used 175c has a damaged bottom right perf area, so also not the most desirable copy, but at least it qualifies.