It seems that that the latest WinSCP assembly v.5.15 from the WinSCP-5.15-Automation.zip was created without a strong name.
I used the following statement at the command prompt:
"c:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.6.1 Tools\gacutil.exe" /i WinSCPnet.dll
v.5.15 gives the following error message:
"Failure adding assembly to the cache: Attempt to install an assembly without a strong name"
v.5.13.8, the same command shows success:
"Assembly successfully added to the cache"