In EASA country, the 350 is one rating with the 300 and 1900
As I understand it a B1900 rating is good for the 1900 and 300 series Kingair, however a 300 rating will not include B1900
Neither of these statements are necessarily true.
I've only ever flown the B200 but was given the BE90/99/100/200 as one rating group and the BE300/1900 as a second. Note the 350 is not included.
I guess it may depend on when your rating was issued as to which ones they included.