The revision v1.2 of the Raspberry Pi 2B replaced the BCM2836-SoC (ARM Cortex-A7; ARMv7) with a BCM2837 (ARM Cortex-A53; ARMv8-A) known from the Pi 3 (Source, German website).
It would seem that the BCM2837 is clocked down on the Pi-2 (compared to the Pi-3) to show more or less the same performance and power consumption as the older revision with a BCM2836. According to this and this the revisions can easily be told apart with info obtained from
What else - beside the switch from a 32 bit platform to 64 bit - has been changed? Are there any relevant or notable hardware differences between these boards?