On Jan 12, 2017, Jim Cheatham in Amelia, VA, USA wrote:
"I had not tried to slide my adjustable steering column until today. It was stuck in place so I assumed it was rust that was holding it. Once freed up, it slid in and out very freely. I thought I would clean the rust off and to my surprise, the splined area of the female section of the shaft is covered with a layer of brass! I'll put a little white grease on it and should be all set to attach the outer steering column tube and the chromed expansion sleeve".

This appears to be copper plating, likely used to reduce corrosion, reduce friction and provide anti-stick joint so it might be adjusted after many years being clamped tight.

The splines will allow length adjustment of the steering column while serving as a positive torque coupling. The band clamp with pinch bolt clamps the outer shaft to the inner shaft for no slip after adjustment.

Inside, mostly out of sight, is a feather key. This is a rectangular key with rounded ends contained in a matching rounded end slot in the outer shaft (held in place by the clamping collar). The inner shaft has a longer slot which allows for length adjustment while preventing the inner shaft from pulling all the way out. If you remove the clamp, pull the shaft to full extension, then rotate it to put the key at bottom, the key can fall out to allow disassembly of the splined joint. This can allow you to pull the short splined shaft out without removing the steering column from the car.

On Jan 15, 2017, Mark Wellard in Queensland, Australia wrote:
"I have two As and they both have the copper finish on the splines. It must be some form of anticorrosion treatment".