The Custom Saber Shop (TCSS) sells a variety of lightsaber components that they call MHS, which stands for ‘modular hilt system’. Buyers can mix and match MHS componets to build a saber. For example, you can select from an assortment of lightsaber pommels, bodies and emitters and combine them in order to construct your saber. Buyers can also easily change the configuration of their saber hilt at a later time, if they so choose.
When you visit the MHS pommel section of The Custom Saber Shop, you’ll notice that some but not all of the pommels say ‘MPS’. MPS stands for ‘modular pommel system’. Pommels that are part of the modular pommel system will have an open end so they can accommodate a pommel insert. Not all MHS pommels have an open end for pommel inserts. Some pommels feature smaller sound holes instead of an open end–these MHS pommels with smaller sound holes are NOT MPS pommels.
In summary, the difference between MPS pommels and non-MPS pommels is that MPS pommels can use pommel inserts, while non-MPS pommels cannot use pommel inserts.