In the 1950s and 1960s, most travel trailers used nearly identical 14" square roof vents made by Hehr. The cover was flat on top and it raised straight up with two screw-type lifters. The lifters were made by LaDeau and it says so on the knobs.
According to the old owner's manual, you're supposed to lubricate the lifters every 6 months so they don't seize up, but no one ever did so most of them are shot. My trailer has three roof vents (6 operators) all the vent covers are bent and the lifters are worn out.
Fortunately, all these vents are pretty much the same. Some have 115VAC or 12VDC fans in them, but most don't. I've been buying used vent assemblies on eBay so that I will have all the parts I need to construct three perfect vent assemblies when the time comes. I've got 4 now.
For 2 good lifters, or 1 good lifter and a a good cover, I've been paying up to $150 delivered. I know that seems crazy, but good original parts are hard to find and spending hours trying to fix a bad one is expensive, too. Here's an example of what I've been buying:
Hehr Vent Assembly Showing Underside w/ 115VAC Fan and Top of Cover
Even after all this expense, there's a chance I may not use them as complete units. Colin and I are working on a way to use a modern Fantastic Vent instead, but to make it look like the original from the outside by putting a vintage aluminum cover on the new vent body. The advantage is that the Fantastic fans are much more quiet and efficient than the old aluminum Hehr fans.