Zener diodes The serial number runs from 10-9999. The third letter indicates that the device is intended for industrial or professional rather than commercial applications. Z. The serial number runs from 100-9999. BC108A, BAW68, BF239, BFY51. Ferranti Examples- ZTX302, TIP31A, MJE3055, TIS43.
Many manufacturers also make custom parts for large volume OEM use.
These parts are optimised for use in a given part of a given circuit. They usually just have a manufacturers stamp and an untraceable number. Often when a company goes bankrupt, or has surplus at the end of a production run, these transistors find their way into hobbyist bargain packs. There is no way that you can trace data on these devices, so they are only suitable as LED drivers, buffers, etc, where the actual parameters are not important. Check carefully before buying.