Can 39-375 and 39-376 fit Pella?

The 39-375 and 39-376 are not designed to work on Pella parts, no. They are only designed to work with Truth brand casement window operators.

I think Truth brand parts may be exactly what you have. That cover you show in your photo may be branded with the Pella logo, but there's a chance that it's not a genuine Pella part. I've seen some Pella windows use Truth operators for whatever reason. It's weird, but it happens. You should check the operator itself to be sure.

I can! That is definitely a Truth operator. I am now very confident that the 39-375 can work for you.