The term originated when the “200” came on the scene in late ’74 I believe. I flew BB102 for a lot of years with many happy memories. The principle difference between a straight 200 and a B is the engines. The former had PT6A –41’s and the latter (current model) has –42’s. No change in SHP but some very minor changes to a couple of wheels to improve high altitude performance.
The 300 is the same airframe as a 200 but has a higher gross, 14,100(?) and more SHP engines…-61’s but I stand to be corrected on that one. The FAA only let Beech manufacture the 300 under a special dispensation for 5 years, 1985 - 90. After that Beech was going to be required to redo the TC. Given the cost to doing that they decided to stretch the fuselage and make it a bigger airframe and then retype it. I am quite sure that the resulting 350 is a FAR25 aircraft. Should note that there were a few 300’s still manufactured after the 1990 cut-off but they are 300LW’s limited to 12,500lbs and were destined for the European market. Flew a 300 for a very short time when they first appeared on the market. Nice a/c, full seats/full fuel and would get as high as 320kt TAS and in and out of 3000ft runways.
Not sure if that answers your queries?