Good morning, Nikon Collector;
That "L39" sounds like something that someone on e-Bay made up trying to appeal to the Leica crowd, or they just plain got it wrong (more likely). The M39 is probably more accurate, but, as many such as E. von Hoegh, jochen, and others have indicated, there are some very real differences in the original thread specifications for the Leica Thread Mount or "LTM" lens thread specification, and the later incorrectly derived "M39" lens specification.
It is very easy to see how a machinist measuring the thread pitch on an Leica Thread Mount lens using a Metric caliper could easily conclude that the thread pitch was 1.0 mm. There is less than one thousandth of an inch difference between them. And most of the places where lenses being made in what has become the "M39" lens mount, were in countries where the metric system of measurements was the most common.
The Ernst Leitz, Wetzlar, company was known for years for their scientific instruments, including microscopes. The common industry standard for the microscope eyepiece thread specifications originated in Great Britain, and the threads were their standard "Whitworth" thread specifications from that time period. There were some differences between the later thread specifications in the fractional inch threaded fasteners here in the United States, and the Metric threaded fasteners in most other countries, but even among the Metric fasteners there were differences, as witness the DIN standards, the JIS standards, and the French and Italian standards, all of which had differences from each other, but they all were Metric "standards;" just for different countries. And now, in an effort to align and harmonize the Metric standards, we have the ISO standards. Being a machinist is not easy.
As jochen has stated, there were some slight differences between the Whitworth specification for the microscope eyepieces whih is 26 threads per inch (TPI), and the very close Metric equivalent of 1.0 mm thread pitch, or 25.4 TPI. The Ernst Leitz, Wetzlar Werke already had the equipment for accurately making the Whitworth threads, so they used them on Oskar Barnack's "Leica" camera using a "double frame" 35mm motion picture film format when it came out.
There was also another difference that is not often recognized; the thread angle specification was also different. The Whitworth threads use a 55 degree thread angle, while the more common Metric and fractional inch threads use a 60 degree thread angle.
So, between the two systems, there could be two (2) differences that could make it "impractical" to try putting an M39 thread mount lens onto a Leica Thread Mount body; the 0.00091 inch or almost one thousandth of an inch difference in the thread pitch, plus the 5 degree angular difference in the thread angle, could produce an interference fit.
While the Germans have for years taken great pride in the accuracy of their machining and finish, the Russians and others could be a little more free with what they considered to be acceptable tolerances. If the Russian tolerances went in the right direction, there might be no problem. That was not always guaranteed.
In summary, yes, there are differences between the Leica Thread Mount (LTM) and the M39 lens thread mount. They are not truly interchangeable.