Many Chinese Pixhawk clones ship preloaded with "ArduCopter V3.6.8 248" — users stick with it because re-tuning for a newer version is time-consuming.
: Boards with the newer "Revision 3" silicon could access the full 2MB (FMUv3). Hobbyists often spend hours troubleshooting why their 2.4.8 won't accept the latest updates, only to discover their board is locked in the FMUv2 legacy. The Ongoing Legacy FMUv3 issue with pixhawk 2.4.8 - PX4 Discussion Forum
This paper details the use of the Pixhawk 2.4.8 for building a DIY drone, covering everything from flight stability testing to mission planning for environmental data collection. Firmware Compatibility Guide