That first character is probably an I, not a 1.
I - International
29 - 29' long (Ambassador, most likely)
D - Double bed plan (generally a gaucho sofa that makes down into a bed along the aisle) This doesn't match with your description of twin beds flanking the aisle, maybe a modification
9 - year manufactured... '69 or '79 most likely
EDIT: the '79 Ambassador was 28' according to Airstream, so probably this is a '69, given the encoded length is 29'.
?? - in this position I'd expect a J or an S (J = Ohio, S = California)
Then the rest is a semi-sequential serial number.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill