Pure manual mode
Just upload the *.jar and *.pom files with paired *.asc for each, in a zip file.
Somehow when the server recieves it they generate 4 checksum files per jar/pom/asc file, making
  total of 10 files per jar or pom.

Using 'mvn' to publish artifacts isn't quite as documented by Maven.
Compared to manual mode, this auto-generates just 2 checksum files per jar/pom/asc file, making
  total of 6 files per jar or pom.
PREP
    If don't have *.pom files, take *.pom files from previous version,
      update contents with text editor.
    Rename all new *.jar and *.pom files exactly as they need to appear in Maven Central.
        I.e. like "hsqldb-2.7.0-jdk8.jar" and "sqltool-2.7.0.pom".
    You must set up credentials for the specified 'repositoryId' in "~/.m2/settings.xml".
    Use browser to verify that new version directory is not present at
      https://oss.sonatype.org/content/groups/public/X/Y/Z  (referenced below as pub version dir)
      !NOW THIS JUST REDIRECTS TO repo1.maven.org/maven2/X/Y/Z!  2024
    Use browser to verify that new version directory is not present at
      https://oss.sonatype.org/content/groups/staging/X/Y/Z  (referenced below as stage version dir)
    Log in with browser at https://oss.sonatype.org/#welcome.
      Select Build Promotion > Staging Repositories in left nav panel and verify that the
      'Staging Repositories' tab that comes up in main panel is empty.
      This tab is referenced below as "Staging Repositories tab".
    Use browser to verify that Maven Central new version directory is not present at
      https://repo1.maven.org/maven2/X/Y/Z
Sample upload commands that work:  1st to upload the base jar and pom files:
  mvn gpg:sign-and-deploy-file -Durl=https://oss.sonatype.org/service/local/staging/deploy/maven2 -DrepositoryId=repotst -DpomFile=repotst-0.1b.pom -Dfile=repotst-0.1b.jar
  The ´repositoryId' value must match  a server <id> in "~/.m2/settings.xml".
  If fails, re-run mvn command with -X switch to see details.
Verify in Staging Repositories tab that a new publish record has appeared (Refresh button).
  Select the publish record to open it in the details panel below the main paneŀ.
  In the details panel's Content tab, expand nodes to verify that these files all appear:
      Specified *.jar file + specified *.pom file + (*.asc PGP file + 4 checksum files) for each,
      for a total of 12 files.
        !NOW CHECKSUMS ARE NOT PRODUCED FOR THE *.asc file so there are a total of only 8.  2024
Then change -Dfile value and add -Dclassifier for sources, javadocs, and custom classifiers, like:
  mvn SAME-AS-BEFORE but change -Dfile value and add -Dclassifier switch:
      ... -Dfile=ossrh-test-1.2-sources.jar -Dclassifier=sources
  Use Refresh button in Staging Repositories tab, detail panel and verify that 6 new files appear
    for each new classifier and the pom (*.jar file + *.jar.asc PGP file + 4 checksum files).
    ! 4 new files 2024.  Plus there are 4 files for the pom file.
  [With the classifier transmittals, the POM gets replaced (uploaded and modified times update),
   though the contents are the same; but the *pom.asc content seems to change (acc. to checksums).]
If you have other products (like for 'sqltool' after 'hsqldb'), repeat base + classifier mvn
  commands and Staging Repositories tab checks for the additional products.  These updates will all
  add to the initially created publish record.
To revert everything (for example if something went wrong and you want to start over), use the
  Drop button in the Staging Repositories tab main panel.
After all uploads verified in Staging Repositories tab, use Close button in main panel.
When the Close operation is completed (see Activity attribute in details Summary tab),
  check that the stage version dir is present and contains the same 4 files * (base+pom+classifiers)
Use Release button in Staging Repositories tab main panel.
When the Release operation is completed, as indicated by the publish record being gone,
  check that the pub version dir is present and contains the same 4 files * (base+pom+classifiers)
Monitor the Maven Central directories.
  The new version directories should appear within a couple hours.
  
Contrary to docs, mvn will transmit via https.