FY, Welcome to the Forum
It looks like the Craftsman # is 315.109020
The part you want is most likely called the lower blade guard.
Unfortunately it appears that part is no longer available,
but check yourself. What I found was at this Site
Call your Sears store. Ask to speak with the PMT. I was one and we were responsible for ordering parts for all store owned equipment and we helped out customers when asked. We had access to all the parts catalogs although consumers do too. A PMT should be able to ask about parts crossover for you.