From the units column, A * 8 must end in 4. The possibilities for A are 3 (3*8=24) or 8 (8*8=64). Let A=3: 633 * 8 = 5064. This fits the pattern 50B4, so B=6. Let A=8: 688 * 8 = 5504. This fits 50B4 only if B is not a single digit. So A=3 and B=6.