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Not sure this is the technology at play here, but ATL Server, a C++ based Microsoft web technology discontinued in 2008 basically supported two files extensions - .srf and .dll. See e.g. https://github.com/redpower1998/ATLServer/blob/bf3fd601bf281... .

IIS (Microsoft / Windows Server HTTP server) supports a technology rival to CGI called ISAPI, which was deployed using .dll files. .dll is a Microsofts' dynamic library standard, commonly produced by e.g. C/C++, but also by .Net.

The thing that stands out the most if the fact that most older IIS setups did not use rewrite on URLs, so instead of pointing to a nice looking path, they were pointing to .aspx files or .dll.

I've only seen it once, and it was deutsche bahn..