Just as a historical aside on the poor man's copyright—
The poor man's copyright is actually an artifact of evidence law, not of copyright law. Under the 19th-century rules of evidence that somehow stayed dominant in this country until the 1950s, a document wasn't independently admissible for the truth of what was contained in it without certain other "indicia of authenticity." I won't bore you with the details… but in the late 1890s, a case in New York held that unpublished manuscripts were the lowest of the low, and that decision essentially spread across the country.
In the 1950s, two things happened: Law professors started to question Wigmore on Evidence, and the Patent Act was revised. The revisions to the Patent Act specifically made laboratory notebooks kept in the ordinary course of research admissible in all patent proceedings, including in court. Other reforms to evidence rules had already begun, such as making books of account (the uncertified, day-to-day stuff) admissible in the 1920s. This, in turn, became the "business records rule"—records kept in the ordinary course of business in support of a legal duty, such as documenting deductions from income for tax purposes.
What this longwinded explanation means is this: Your file copy of the manuscript plus your regularly kept submission log are admissible evidence of the provenance of your manuscript now, whereas they weren't a century ago. Given the speed with which things change in the publishing business, it's not entirely surprising that the myth of the "poor man's copyright" persists. The sad thing is that even the poor man's copyright didn't do the job… because the postmark on the envelope couldn't necessarily be accepted as proof of its own validity under the evidence rules of the time, since the item as a whole was not a government document!