The designation of CPU speed for a 486. The second number was the clock rate: 486/50 meant 50 MHz. CPUs beyond 33 MHz may have had different internal and external speeds as noted in the following table. The internal clock speed was the rate at which the CPU processed (calculated, compared, etc.). The external clock speed was the rate at which it communicated with RAM and external bus.
486 Type Internal External
486/25 25 MHz 25 MHz
486/33 33 MHz 33 MHz
486/50 50 MHz 50 MHz
486/50DX2 50 MHz 25 MHz
486/66 66 MHz 33 MHz
DX4/75 75 MHz 25 MHz
DX4/100 100 MHz 33 MHz