BMW used two completely different spline counts on original E9 steering box input shafts, depending on whether your car is equipped with Power Steering or Manual Steering (including the CSL).
-Standard Power Steering Boxes: These original units utilize a coarse spline input shaft (typically 36 splines).
-Manual Steering Boxes (and CSL models): These factory mechanical boxes feature a fine spline input shaft (typically 54 splines). If you are replacing or reattaching the lower steering column spindle, the u-joint coupler, or a solid steering shaft upgrade, the splines will not match if the replacement part was sourced from an E9 with a different steering configuration than your BMW E9 CS steering boxes use a coarse spline design, while later BMW steering wheels and columns (post-1985) use a fine spline design, causing mismatch issues. If your spline teeth counts differ (mid-30s for coarse vs. low-50s for fine) or if you are using a non-E9 column, they will not fit.
Coarse vs. Fine Splines: E9s use coarse splines (approx. 36 teeth); later wheels (like E24/E28/E30) switched to fine splines (approx. 50+ teeth). 
Column Changes: BMW changed the column spline design over time, meaning later steering wheels do not fit early columns without changing the entire steering column.