This is a Rectify Series drop-in speaker system for Walthers Proto E-Unit locomotives (and possibly late-run P2K). It has a slight oblong egg-shape that fits perfectly within the frame's cavity and a domed top following the roof's contours. 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. Usually used in pairs, but can be used singly as well. No modifications are needed to replace the factory speakers.
 
The 21pin-equipped versions I have seen use the "small" 1.0 2-wire speaker harnesses: https://www.scalesoundsystems.com/product-page/factory-pcb-speaker-harness-small
 
The speaker comes fully assembled and tested.
 
Dimensions are: 30-32mmDia x 14-16mmH