Nimschke signed near every piece he did. The elaborate with a full name, some with 'LDN' in caps, the lesser ones with a simple capital 'N' done with three quick
swipes of the graver. Since he was a freelance engraver and one of very few to really self promote his name and engraving, he took every advantage to mark his work. Firearms manufacturers usually did not want him to sign or mark his work in any way ( a few did), nor did they normally want their 'in house' engravers to either. Nimschke usually looked passed those demands and found a way to mark them anyway.
Attributing unmarked engraved pieces without documentation to Nimschke, or any other engraver for that matter, is more of a guess on the part of the 'expert', than having anything to do with being able to examine the cutting and collecting hard facts from it. There is nothing magical or signiture in the examination of cutting style or marks that can lead to a identification of a specific hand. Tools change, get sharpened, time/age/injury/ailments changes technique. Engravers styles & quality evolve as they procede through their career, perhaps doing a down swing at the end. Engraving cuts haven't changed at all and the combination of cuts to produce certain patterns is a common a language to engravers. You do find a few little self learned tricks that you later find out were already known by the rest of the world. You do develope your own style just as you do your own handwriting but copying someone elses style and work is not impossible. More tedious for sure, but not all that difficult though most engravers prefer to do their own identifiable style of scroll and work. Nimschke's most popular style as shown is, for all it's beauty, quite easy to both lay out and cut. It is fairly fast to cut and fills the area quickly. It was copied and used by countless engravers then and still is now. Copying a popular style was and is nothing new. Even signing the Masters mark to it to lend a bit more to the value.
When Lynton McKenzie was at the top of his engraving career in the US both in skill and most importantly popularity, more engravers copied his style of scroll work than any other, with the possible exception of Nimschke again. It became known as McKenzie scroll. The style and the value of the work & name attached to it became such that you could attend a big money show such as LasVegas and see more 'McKenzie' engraved guns than the man could have ever have done in his now unfortunately shortened lifetime. Many an expert in engraving could not pick the best of the counterfits from the real ones. They should sometimes have looked at the gun it was on more than the engraving to get a hint of it's originality.