Airstream keeps moving the Secret Decoder Ring around on their website. Here's where you can find it these days for older models:
Airstream, Inc :: Serial Numbers
What ya got there is (Click on the link labeled 1969 to 1980 diagram at the top of the page): 027DOJ118
O for Overlander
27 for the length of the trailer
D for Rear Bath/Double Bed
0 for the year it was built (1970)
J for Jackson Center (where it was built)
118 Sequential serial number
The "O" for Overlander in the first position is not on the page, but that's what it really means, all 27 footers that year were Overlanders.
Jim