Re: mauser-nickl pistols
Sorry, I'm not all that familure with Nickl's designs. Given that I have no idea of how your non-ratating barrel example works, I would have no idea if it was a novel design or not.
I suspect that when it comes to such rare firearms as yours, there may not be much if anything writen about them. For this reason, you may have to do some primary research of your own. I might suggest looking for Nickl in German patent records of the period. Before you do that, you might like to send some good quality images to the NRA museum in Washington D.C. With all of the data they have on firearms, there is a good chance that someone will know what you are looking for. To make it easy on yourself, I suggest sending your request in both English and Spanish.