I've seen the C4 Monza identification too, and it is utterly wrong. The C4 Monza was a sportscar built in 1924 based on the C4b pictured below. The source of the wrong identification is probably an erroneous caption in a well known Swedish veteran car blog, where the C4 Monza caption was used twice, once correct, and once in error for the puzzle car.
Further on, the K4 and C4 were different cars, despite their similar engine data. The K4 still had the cylinders cast in pairs, while the C4 had a newly developed engine. Schrader shows a pre-WW1 Typ K4 with a veed radiator, so this fashion must have reached the final prewar models already.
Finally, almost all my books claim that NAG's passenger car production was stopped in 1914 and only resumed in 1920, which makes the 1914-1919 production period (which only Schrader has) doubtful.
