This is a Rectify Series drop-in speaker system for current-run Athearn Genesis and RTR locomotives that come with the "dual-cube" speaker or weight. The speaker uses the factory mounting screws to mount to the plate and no modifications are needed to replace the factory speaker or weight.
Note that while this will drop-in to the current RTR SD40-2s, I do make a specific speaker for these that replaces the stock plate and sounds a touch bigger: https://www.scalesoundsystems.com/product-page/athearn-rtr-universal-sd39-40-2-current-run
The speaker comes fully assembled and tested.
Dimensions are: 13mmH x 17mmW x 38mmL