The original idea for the A300 was a larger aircraft, to seat about 300 pax and be powered by two Rolls-Royce RB207 engines. This was dropped in favor of the A300B, which was around 250 in capacity and powered by the CF6-50. At that time, there were many variants proposed, all with a number following the B. The first two airframes completed were A300B1's, the third was modified to the A300B2 standard, with a slight fuselage stretch. The higher-weight version was the B4
. Other ideas included the B9, with a much longer fuselage, the B10 with a shorter one, and the B11 with four engines. These ultimately became the A330, the A310, and the A340 respectively. The earliest models had a simple designation, like -1A, -1C, -3A. I have never seen an explanation for what these designated, although it might have been engine variants. As others have stated, the B2K had the inboard Krueger flaps in common with the B4
. Airbus later adopted their present nomenclature, with the original B2
becoming the B2
-100, the B2K now the B2
-200, the original B4
-100, and the heavier B4
-200. In addition the last two digits now designated the engine variant, the first the manufacturer and the second the actual model.