A quick learner from USA says:
September 8, 2016
Our Pella windows were installed in 1991. We need to switch the handle from the traditional crank to a T-handle. Our Pella tech told me that the our handles are 11 (12 to be exact) splines and 5/16
Dave from SWISCO responded:
September 9, 2016
Pella casement operators almost all use a 3/8" spline. The official Pella brand T handle we carry is the 39-227B. However, you say yours is 5/16". Are you sure this is a Pella window? Would you be able to show me a picture of your existing operator? That may help us find something that matches. I also recommend double checking the size of your existing spline, just in case.