The versioning of Flex can be somewhat confusing for new users. The release is widely considered the last major release of the "classic" Win32 port era and one of the most stable legacy versions used in academia.
FLEX 2.5.4A COPYRIGHT (C) 1980, TECHNICAL SYSTEMS CONSULTANTS Flex2.5.4a Download
If you must have the exact version:
And finally, the Flex command prompt: