Mr. Maico 75,
I have studied the work of the worlds engravers for over 30 years and in particular that of Louis Daniel Nimschke. If you will go to this page of my glossary you will find a brief biography of LDN:
http://www.engravingglossary.com/Han...ossary%20N.htm
LDN was active from about 1850 to 1900 though he was already a trained engraver when he immigrated from Germany. LDN did not sign many of his works as it was not the custom at the time to do so so. You will only find his name or "LDN" or LDN Eng" on his most elaborate or unusual guns and also on the hundreds of fakes out there. In fact, a Nimschke signature on any gun without strong provanance, especially if it is not highly ornate, should be considered a fake until proven otherwise.
It is my opinion that the engraving on your Sharps is contemporary with the manufacture of the gun. It is also my opinion that it is not by LDN though the style of scroll is very similar. Your conclusion that early in LDN's career others were not copying him is correct however the style of scroll that he is best known for was not entirely unique to him and was in common use by German trained engravers.
There are certain telltale signs that I look for in work that was possibly done by LDN that I will not go into here. In any case it would be worth your while to send the pictures to JRon and let him do some more research. If I had the gun in hand and could examine the cuts under high magnification I could give a more positive assessment but with what I have to go on I do not think it was done by LDN.
CRB