I am not 100% certain, but I believe the Argosy used the same lock as Airstream, and that was the last year for the KT Lock VTS-1318.
Here is my blog post on that lockset: http://beahmstream.com/1972-airstrea...lock-vts-1318/
Unfortunately that is a really expensive replacement if you can even find one. When they were in stock, they were $500 on Vintage Trailer Supply. Those are all gone, and they go from $750 up on eBay. As mentioned in my blog post, it's often cheaper to buy a whole door (or even trailer!) rather than just the lockset.
Hopefully others can chime in on decent replacements, as I have always gotten lucky and found originals for my builds. I've got a 1976 Excella off the frame, and it's missing the lockset, so I am interested in a good (hopefully *better*, as those KT's have their issues) replacement as well.