In case anyone wants to replace the original static ventilators that came on the 1980 H33, I found that the Beckton (cheap, about $24 dollars) with a plastic cover is the same internally as the original. You can even use the stainless cover from the old ones instead of the supplied plastic cover. The screw holes are the same, so no new holes need to be drilled or filled, and they fit in the orginal openings. Took about 1 hour to remove and install all three. Must have had good service from the old ones, all of the balsa sandwhich was bone dry.