need help with broken knob stem

Thanks so much for reaching out to us. This is a very old style Fenestra balance. They haven't made this in a long time, unfortunately, and there's no verified replacement.

Only one product comes to mind that may replace this, which is our 39-021. This is also used for old windows, though admittedly not as old as yours. I think if you're handy and if you're not afraid to do some serious retrofitting, there's a chance this can work. Look it over and see what you think. If you aren't handy, then you may want to consider hiring a professional to do the install for you. At minimum you're looking at redrilling holes but some alteration of the sill and sill trim may be necessary. Be prepared to improvise.

We also sell the 39-020 for the opposite side and the 39-504; the latter of which will be necessary.