The first N-frame was called 38/44 Outdoorsman. It was a 38 Special mated to the 44 Hand Ejector frame. Creating a large, heavy, wonderfully accurate 38 Special for heavier loads. The 357 Magnum was a pretty direct decendant for use in the heavy frame. These were the 'Registered' 357 Magnums.
Both are highly collectible. The N-frame has been used for calibers from 38 Spl to the 44 Mag.
I beleve the Triplelock or .44 Hand Ejector 1st Model is the first N, coming out in 1907. It was supplanted by the Second Model which did not have the crane lock or shrouded ejector rod in 1915. The shrouded rod reappeared in 1926 with the 3rd Model (Wolf & Klar) .44.
The Outsdoorsman (1931) and Heavy Duty (1930) were the first .38 N frames.