Suggest you talk to a Dometic dealer. It is getting hard to find reference to the old style awning arms used back then. I have an old catalog that shows the various arm shapes/sizes. I'm not sure if they still make all of these.
The awning arms are on page 33 & 34. Look at the mounting instructions on page 37. The main consideration is the measurement from the floor of the trailer to the upper mounting point (which should be centered on the mounting rail. The awning arms show a range that the arm fits.
Arm 015 is listed for Airstreams thru 1985 (Min 75-5/8 Max 81-5/8). That is too long for my 58. I ended up buying a set of these arms on Ebay (freight damage, coating scratched) and shortened both ends to fit. Not an easy solution.
Hopefully a dealer can find you an easy solution.