The Athearn Roundhouse (now branded simply as "Athearn") GP40-2 uses the AHRR-UNI1 speaker. Models that did not come with a speaker will require screws and you have the option of adding M1.7x6 self-tapping screws to the order. Be sure your model has the two mounts behind the motherboard.
 
More than just a box, the enclosure features Diapasonic Tuning™, which allows the lower frequencies to accurately develop and resonate, clears the midrange of excess resonance and tunes the upper-mid/high response. The speaker uses the factory mounts and no modifications are needed to replace the factory speaker (or install this one on models with mounts but no speaker).
 
The speaker comes fully assembled and tested.
 
Dimension: 15mmH x 18mmW x 43mmL