I believe the source code of the DLLs for the old Windows SDKs such as '2016 R2' and '2016 R3' were not publicly available, as those SDKs were not fully open source and some of the DLLs were considered to be the intellectual property of Intel.
The '3DScan' 3D scanning program does have C# source code though. In the '2016 R2' SDK it can be found in the following directory location:
C > Program Files (x86) > Intel > RSSDK > framework > CSharp > DF_3DScan.cs
Alternatively, you can find the source code by locating the 3DScan sample in the Sample Browser program supplied with the SDK. the Sample browser is in a folder called 'Intel RealSense SDK Gold' that is placed on your desktop when you install the R2 or R3 SDK.