Should be just shy of an interference fit. We some times have to bend the tabs out slightly (whack with a plastic non marr hammer), position the tub and then another whack, to avoid scratching new paint. I use a thin nylon washer between the steel frame tab and the aluminum body work in an attempt to prevent the galvanic corrosion that eats the tubs behind those tabs.
BTW I have two pair of galvanized bumoerettes like those pictured if anyone is looking.
BTW I have two pair of galvanized bumoerettes like those pictured if anyone is looking.