I should have thought of this sooner.
I made a copy of my .sln file.
I did some "unchecking" in the Configuration Manager (under the menu item "Build"/"Configuration Manager")
I saved the .sln
I did a winmerge comparision with the copy.
The magic line seems to be "Build.0".
Before
{BBBBBBBB-AAAA-1111-2222-333333333333}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
{BBBBBBBB-AAAA-1111-2222-333333333333}.Debug|Any CPU.Build.0 = Debug|Any CPU
{BBBBBBBB-AAAA-1111-2222-333333333333}.Debug|Mixed Platforms.ActiveCfg = Debug|Any CPU
{BBBBBBBB-AAAA-1111-2222-333333333333}.Debug|Mixed Platforms.Build.0 = Debug|Any CPU
{BBBBBBBB-AAAA-1111-2222-333333333333}.Debug|x86.ActiveCfg = Debug|Any CPU
{BBBBBBBB-AAAA-1111-2222-333333333333}.Release|Any CPU.ActiveCfg = Release|Any CPU
{BBBBBBBB-AAAA-1111-2222-333333333333}.Release|Any CPU.Build.0 = Release|Any CPU
{BBBBBBBB-AAAA-1111-2222-333333333333}.Release|Mixed Platforms.ActiveCfg = Release|Any CPU
{BBBBBBBB-AAAA-1111-2222-333333333333}.Release|Mixed Platforms.Build.0 = Release|Any CPU
{BBBBBBBB-AAAA-1111-2222-333333333333}.Release|x86.ActiveCfg = Release|Any CPU
After:
{BBBBBBBB-AAAA-1111-2222-333333333333}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
{BBBBBBBB-AAAA-1111-2222-333333333333}.Debug|Mixed Platforms.ActiveCfg = Debug|Any CPU
{BBBBBBBB-AAAA-1111-2222-333333333333}.Debug|x86.ActiveCfg = Debug|Any CPU
{BBBBBBBB-AAAA-1111-2222-333333333333}.Release|Any CPU.ActiveCfg = Release|Any CPU
{BBBBBBBB-AAAA-1111-2222-333333333333}.Release|Mixed Platforms.ActiveCfg = Release|Any CPU
{BBBBBBBB-AAAA-1111-2222-333333333333}.Release|x86.ActiveCfg = Release|Any CPU
That's what I found after 20 minutes. Maybe it can help a future reader, so I'm leaving the question up....