Welcome Argonaut. At least in '75, Argosy trailers had the VIN stamped on the top flange of the A-frame between the body and the hitch coupler, as esmaytum describes.
You can guesstimate the model by measuring the distance from the bumper to the hitch. Looking at the photo you've posted, I'm guessing 26 since from the placement of the windows it seems like it had the galley on the curbside, it's a tandem axle and definitely seems longer than 22', thought the outside photo is at a big angle off the side view.
See if you can find the VIN on the A-frame if there's not a VIN tag anywhere... there's definitely not one where mine is, above the fresh-water filler door. Does it have a license plate? (i.e. do you know the state in which it was last registered?) If it has a plate you can probably find the VIN that way, but I wouldn't be 100% confident until I found the VIN stamp, since someone might have given the wrong VIN to get the plate, etc.
Zero Gravitas — 2017 Flying Cloud 26U | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill