It changed the name of the album in MC to..... 1900 bla bla (+1900) and I didn't like that look in MC
I too would have used a separate Custom Field in MC, as it would work faster in Views, and be simpler to understand and use, for future maintenance.
As Ferday said, you can review what the "Fill Properties From Filename" function will do before letting it do anything, and can test it on a small batch of files, then run it over your whole Library very quickly.
If you had used the Template functionality in "Fill Properties From Filename" instead of the Automatic functionality then you would have had complete control over the naming in MC.
For your example directory name:
1965 Frank Sinatra September of My Years (+1966)
You would use a Template of;
[Year] (+[GrammyAwardYear])
I created a dummy file called "10 - Frank Sinatra's Best - Frank Sinatra September of My Years.mp3" to show you how easy it is. See the attached image.
Note in the images;
I used the existing [Date (release)] tag rather than creating a Custom field [GrammyAwardYear] for this example.
I made up the Album name and Track #.
The first image shows just a simple version of the "Fill Properties From Filename" dialogue that extracts just the [Year] and [Date (release)] tags. That is all you need if your other tags are all correct.
The second image shows a more complex version that pulls out more data, if required.
Unfortunately, your album folder name may include both [Artist] and [Name] tags, but doesn't include a separator between them, like a "-" or similar. Easily human readable because you recognise the name "Frank Sinatra", but not so easily recognisable for software. So I have just left the [Name] tag as "Frank Sinatra September of My Years" in the second example image. I'm sure Brian could write some Regex to split the two, if there were any consistent rules to your folder and file naming.
Or if your file name was just something like "10 - Frank Sinatra's Best - September of My Years.mp3" then the [Album] and [Name] tags could easily be extracted.
Using MC the way it is intended, rather than structuring folder and file names, then having to pull them apart to View, always works out better in the long run.