This may help
Not 100% but this is my understanding
The 1900 was certified under SFAR 41 which is an adaptation of FAR23 so that some aircraft can meet most of the requirements of FAR 25.
Reasoning behind this was that the 300 was just a heavy 200, and the 350 was a slightly fatter version of the 300. The 350 formed the basis of the 1900C which then morphed into the 1900D.
All the time the aircraft remained single pilot capable (like the Twin Otter is a sp machine and the rules say 2 crew for commercial ops). With this in mind the rules allowed the aircraft to be operated with 1 crew should the pax seating not exceed 9 (?).
A few years back an operator in OZ flew their 1900’s by day two crew with pax on board and by night with cargo single crew!
The same with the Metro III/23 this thing is certified SFAR 41 and grandfathered on the type certificate of the Merlin/Metro II which was mtow 5700.