Well, yeah, if there's nothing holding it in the MC it can just pop right out. Have you ever pulled out a MC and had to remove the pushrod? That thing gets jammed in there pretty good when the retainer is used.
I don't have the part number, I went to AutoZone and asked for an aluminum MC for a D100 truck. They pulled out a couple, I checked the back to make sure it had a little recess inside of where the pushrod goes (for the retainer) and got one for around $50. New, not reman, and from AutoZone it comes with a lifetime warranty. Make sure that there's a retainer in the box, too. It's a small ring... here's a pic: