i thought this would be fairly easy but it took me some searching to find a date on my 1300.
first off i couldn't find a riveted plate with any info in any spot to the left of the door.
On the front of the trailer just above where the regulator is mounted I have a foil sticker that has a VIN
# on it and also has a date of manufacture(ours is a 1974). If you need I can get a picture posted in the next day or two.
there is a production number on an info sheet stuck to the inside of the closet, but nothing in that number suggested at date of manufacture to me.