Hi Dave!! I would have to say the collectability of a 41 Code 42 versus any other Military Mauser depends on what one defines as difficult to find. If one examines the total production of a given major variant then the 41 Code 42 will be the most desirable because the estimated production was, as Ed says, 7,000. Quite a few less than a K-Date at 11,000.
However, you also have to ask yourself how many of each are available to be purchased. K-Dates were made in 1934 and 41 Code 42 were made 7 years later. K-Dates were involved in about 6 years of war; the 41 Code 42 in only 4 years of war. How many survived the war and how many are still available?
If one examines the different sub-variations, then you would come to a different conclusion. As a for instance, the 41 byf with mixed Waffenamt marks SE/655 & SE135 are very rare. In fact, so rare that the data is sparce enough to preclude even a good guess as to the quantity available, but certainly less than 500, many estimate less than 50.
If one examines the sub-variations of K- & G-Dates, you can find production quantities of significantly less than 7,000.
While the 41 Code 42 is a very desirable Luger Variation, I think there are other variations that are more desirable.
Anyway my 2�¢ worth