rigid spline like forever but it is slightly smaller

They only made the 55-109 in one size. You might want something like our 59-241, which is narrower but flexible. If that won't work either, though, then I will need to know more specific information about your old spline. I would like to start with pictures (taken from the same angle we take for our own spline, so that we can see its shape) and detailed dimensions. Thanks!