So I bought a new air filter, a uni foam filter, pop up the seat, go to switch 'em, pry the old element out (which is was shockingly difficult), and find... nothing. Literally, nothing at all in the airbox. PLUS, it's a 74 550, and the airbox is marked "CB550F". Neither the element I pulled out nor the one I purchased for it appears to fit properly in the box without having something under it...
So, my thought for quick, get-it-back-on-the-road fix was foam, or a sponge, or something under it, and a spring on the side of the box nearest the rear-wheel to hold it in place. Seems like that would work, but it's also not proper, so what exactly am I missing? Anywhere to get an entire air-box setup? Who's got pods and doesn't need their airbox?