Okay, I had a bit of sticky beak and found this thread on the ebay boards where a poster (who is apparently a qualified lawyer or somesuch) quotes a particular law. You can see the thread here:
http://forums.ebay.com/db1/topic/Vintage-Clothing-Accessories/Clues-To-The/510114554
The relevant passage is here:
The "Reg. U.S. Pat. Off." (Registered with the US Patent Office) label was the precursor to product registration (the R in a circle) under the Lanham Act (also known as the Trademark Act) of 1946.
The law took effect 1 year from enactment, and so became law in 1947. During that time period, manufacturers were registering their names and lines, as well as their trademark names and logos. After July 5, 1947, a garment would have had a label with the brand name with an R in a circle, instead of the "Reg.U.S.Pat.Off" designation.
Now, I don't know about that particular bit of legislation (you can read the Wikipedia page
here but it doesn't mention the garment industry so I can't be sure it even applies) but I do know a thing or two about vintage clothing:
1 - the dress on the ebay thread is not pre-1947. It is '50s.
2 - I agree with Jonathan and Mary that your dress is 1950s.
3 - labels can easily be removed or added to a garment so they are never proof of anything.
4 - a fashion company may use a label for a longer time than expected.
5 - labels are a useful tool to date a garment but they are only one piece of the puzzle: other pieces include style, fabric, construction techniques and detailing.
You know, I'm sure the ebayer meant well but I wish that people wouldn't talk in absolutes when it comes to dating vintage because there are very few absolutes. It's important to piece the clues together and come to a likely date. Even here at the VFG where there are so many experts, there's not always agreement, as a garment holds so many clues, sometimes contradictory, like your label.
Nicole