The reason I suggested using pdinfo is that they actually keep documented copies of songs that show that they are in the public domain. As I said, they have discussed Happy Birthday, and information about it is available there.
By clicking on the User Name, you will requery the forum for that user. You will see everything that he or she has posted with that Mudcat name.
By clicking on the Thread Name, you will be sent to the Forum on that thread as if you selected it from the main Mudcat Forum page.
By clicking on the Subject, you will also go to the thread as if you selected it from the original Forum page, but also go directly to that particular message.
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sj