They're part of the NI Measurement Hardware DDK. We have a description format that we use to describe the registers available on a hardware board, and generate C++ accessor classes based on those. Some input files are distributed, for example, in the X Series DDK... they're the "rbm" files. It doesn't appear, at a glance, that we distribute the rbms for the chips on older products-- so all that's available is that output, sorry.
They're really just the C++ form of the information available in the DDK manuals. For example, tSTC.h is just an implementation of information from the DAQ-STC Technical Reference Manual.
If you have more questions, I'd recommend the DDK forum, or perhaps the DDK Programmers group. (Not everyone watches every forum, posting there might attract the attention of someone more involved with the DDK than I am.)