Not sure what brand window this is

Thanks for posting. Don't worry too much about the manufacturer--many of these parts are generic and used by a number of different brands. Your casement operator, however, is a bit curious. At first glance, this casement operator looks akin to our 39-031 and 39-032, so long as the dimensions of the arms match what is shown below. Let us know if anything differs.

Dimensions for 39-031

However, the 39-031 and 39-032 have an operator base that has the spline the handle attaches to perfectly centered, as shown in the graphic above. The handle looks offset in your photos, but it may just use a folding cover handle like our 39-107FK, which works for centered splines. Check the positioning of the spline on your old operator to confirm if this is the case. If not, let us know and show us photos of what's under that cover, and we'll try and get to the bottom of this.