The A/C is definitely newer than the trailer.
did put out models set up for air conditioners- the mod was a roof mounted molded fiberglass cap that was designed to provide the extra roof support needed. Can't tell from the pic if this trailer has that asset or not, though the inside pics don't seem to show any interior sag.
Be aware that there's a frame recall out on 1300's from 1973-76. Link to recall
Fix is relatively simple, requiring welding on of reinforcing plates. Can't tell from the pics if it's been been done on this trailer frame....not a deal breaker I guess but if you buy it and it is that age range, do see to it for your own safety!