I got a 75 Donnell that has a sticker on like you're talking about. It's faded from the sun and you can't make out much of what it says. When I had it registered in 95 from out of state, the DMV did an inspection, saw the sticker  was faded and said I needed to have the "VIN" number stamped onto the trailer somewhere.

If the number is stamped on the trailer, I don't think you have anything to worry about. Let us know how it works out.