Looks like you have an older style operator. They used to make these with removable covers, but that's not the case anymore. Most modern operators like yours have the cover built into the operator itself.
To confirm, can you show me a photo of your complete operator out of the window? That would allow us to identify which of our parts can replace yours.
Ah, this old thing. I'm afraid we don't have anything that can work for this. Your operator hasn't been made in a long time, and as far as I know nothing we have can be used as a direct substitute. The 39-323 can't be used as a cover, either; it's identical the the 39-107 you already tried, it's just a different style.
You could maybe use something like our 39-027E or 39-013 as a substitute operator, but there are no guarantees either will work. Their bases will be a lot smaller than yours so there'd be a gap in the sill trim. I'm sorry that we can't offer a better option for you.
