20 year old white Norco encasement window replacement T handle

The size of the spline is important, so you're going to have to get a more accurate measurement. It's either 11/32" or 3/8", but we can't decide that for you.

This looks to me like a Truth brand operator. If it is, then 11/32" is standard for their hardware. In that case I'd recommend the 39-102, but you need to be sure of the size.