The iMac was tray-loading all the way through the Rev D model. There were two revisions of the Bondi Blue iMac (233MHz) (Rev A/B) and then two revisions of the first batch of multicoloured iMacs (Rev C/D, 266MHz and 333MHz). All of these were tray-loading. It was only after this that the slot-loading iMacs were released.
The original ‘Bondi’ iMac was released in 1998. The last of the tray-loading iMacs (Rev D) was released in 1999. For reference, here’s a photo of my ‘Blueberry’ 333Mhz Rev D iMac from ‘99.