The Sound Devices MX-LM1 is a battery sled designed to hold one Sony L-mount battery (available separately).
This can power units in the original MixPre series, the MixPre-M series, and the MixPre-II series.
With most compatible recorders, releasing the MX-LM1 is as easy as pressing the release tabs. However, on the MixPre-10 II, a flat head screwdriver will be necessary.