18 spline aluminum case 4 speed

When I asked the fella if it was an OD unit he said it was not. I am waiting for a call back as there is another guy ahead of me. If/when I check it out I can always spin the input and in 4th its one to one, so that would rest easy on weather its an OD, but doesnt it seem wierd to have an 18 spline in aluminum? or am I over thinking it?