There are a number of plate flaws on these Vurtheim issues, but this looks like a bit of dirt lodged in the plate.
All these Vurtheim designs were forged by unauthorised reprinting from the original plates. There are forged surcharges on these forged stamps.
There are a number of ways to check whether the stamp is a forgery. Have a look at the gum. The forgeries have whiter gum and thinner paper than the original issue. When viewed from the back, the impression on the reprint is quite clear because of the thinner paper.
The colours tend to be more vibrant on the original issues, but since there were three printings of this 3d value you can always expect a certain amount of colour variation.
The perforations on originals of this surcharge are comb 12-1/2 x 12; reprints are all line 12-1/2.
The surcharge itself looks OK when compared to the known forgeries.