The fixture/jig holds the trigger for one step in the operation and the sear for the other and gives you a guide set at the proper angle(s) for the stones to glide across. A friend had one for 1911s that I used on one of mine. Worked great, only cost about $35.00, so they stopped making them.
I did my other 1911s with just my knife sharpening oil stones and they came out just like the one I had use the jug for. The trick is to keep everything square and not to take off “too much” metal. Once over done it’s time to replace parts.
Marlin customer support Phone #: 866-662-4869. Wouldn't hurt to give them a call to see if they’ll sell you the parts. If they won’t I raise a ruckus.