The term "date of return" is a legal term that means the date on which a legal document was received back by the government entity. It's essentially the date on which there was evidence of recording back to the government entity. There are some times legal rights and obligations that trigger off that date.
The "odd will" does isn't the will. It's the letters of administration, which appoint a person or persons the administrator(s) of the estate. It's one of the initial steps in most probates.
As George said, you need the original records of the probate proceeding. There should be documents that list the possible heirs, such as the will, a decree of distribution or a final decree. Sometimes they have been destroyed or lost, but they are generally there.