23 spline to 18 spline

I think I have only ever had one 18 spline A-833 apart. And it was a coarser gearset. And I have read dozens of papers that say 18 spline trannies are a coarser gearset.You need to get an entire gearset as a pkg.You cannot mix coarse with fine. I have several empty cases that I use as door stops and spacers.
I have had hundreds of 23 spline trannies apart and every single one was a similar cut gearset. Im not saying it ( an 18 spline fine gearset) doesnt exist, only that I never saw it in nearly six years of building boxes on an assembly-line, from 76 to 83.
Or did I completely misunderstand your question?
in which case,Why would you consider buying a box of loose parts to fit into your case? What am I missing?