I saw on your post about the front window...it also has (I think) the rounded corners on the kitchen cabinets..?
1959 is the last year they built those cabinets like that. Check the bottom side of the sink. There may be a date stamped on it, which would indicate the manufacture date of the sink. This would tell the earliest the sink could be made and usually the date of the trailer manufacture is pretty close. This same tip goes for the mirror as well, many had date stamps on them too.
You will probably get a better idea also when you hear from Ross, our resident VIN expert...he has built a database of VIN numbers and can narrow the age down by comparing to other VIN's in the database.
This is really common. There is quite often no clear lineage on the old ones, that have either sat till the original owners are long since gave up on them, or they have been passed around way too much for anyone to remember where they had come from. It only takes one lost registration in a transaction to lose the definition of its heritage. Someone makes something up to register it home-made, and then it comes to detective work to figure it all back out.
If you want some color variation you can get different colors of shellac if you stick with the birch look. I'm thinking of keeping the ceiling a clear shellac for a very light color. The walls just hint of yellowish brown, so just a shade darker than the ceiling and then a deeper brown for the cabinets.