It looks like one of his to me. But without a signature it can only be attributed to him. Nice gun though. What is the asking price and how are you trying to go about selling it?
I would contact Linda Kohn/Sherwood and/or Brian Lebel of "High Noon Auctions" 1st.
Then I would holler at the Gene Autrey museum in L.A.
The High Noon auction has sold several P.V. possessions over the years, realizing top dollar.
From the pictures that you have shown, I am about 90% sure that it is a genuine LDN engraving. I would like to see close-up images of the side panels and one of the cylinder facets to achieve any certainty.
I have been asked to authenticate many purported Nimschkes, most of which were fakes. A signature is no guarantee of authenticity as the majority of spurious Nimschkes have some form of forged signature. In fact, when I see a signature on a purported Nimschke, I am immediately suspicious since LDN only signed about 18% of his work, usually the most elaborate. There were also a number of 19th century freelance engravers who copied LDN's style but there are discernible differences because those engravers were not trying to fake LDN work (he wasn't famous then). They were just working in a popular style.
I look at possible LDN engraving the same way as a police criminalist looks at fingerprints, using multiple points of comparison.
We often forget that what to us, today, is an example of great work from the past was, to the engraver, just another production job to get whacked out ASAP.
But as they say, practice makes perfect and cranking out piece after piece after piece tends to up your game. Look at Sam, to pick just one of our fellows who have been doing this for a living long enough to, shall we say, get the hang of it.
R.L.Wilson and John J. Malloy put together a wonder reference of smoke pulls on the engravings of L.D. Nimschke,with out this book and men like R.L. Wilson John Amber and James B. Meek who promoted and educated us on this wonderful art we would not be here on the two forums exchanging ideas and learning from each other. J.J.