Missing bottom piece of my window

Hi Sally. Thanks for writing in. Hm, I'm not sure if part of the frame broke or not. It doesn't look like it, as it looks like a very clean hole. Could you confirm this for me?

If nothing broke, then something is missing. Could you check an identical window in the house that may still have its hardware? If so, we can use that as a reference. Many thanks.

Thanks for the reply, Sally. We're getting there! I'd really like to see the pivot bar out of the sash, if that's possible. Looking at this photo, I am reminded of our 26-424, which has a similar design, but I'm not sure about the rest of it. 

Now, that only covers the pivot bar. We can't replace the piece missing from the sash that holds it in. I am afraid that is not something we can replace. To be honest, it looks like they have a separate plate or something that performs this function. Could you confirm? Many thanks.

That sounds like the plate I was talking about. I would check with a local hardware store to see if maybe something could be fashioned to work in the same capacity. It doesn't look too complex, so hopefully a local professional could whip something up for you. Sorry we couldn't be of more help!