These look like they might be data/index files from CSAM or one of its derivatives. If so, the individual records appear to be small with a high degree of repetition, like what you'd find in scientific or manufacturing measurements where the values changed very little from one reading to the next.
If that is the case, you'll need to figure out what system (or at least what ISAM library) created them, then look for a .NET data access library that can process files created by that ISAM library.
-PatP