Master cylinder identification?

Another brake mystery. While I was dealing with the master cylinder on my bike which is having problems, I took a look at a spare master cylinder that came with my 75 MK3. Externally identical to the one on bike except...when you look into the bore the first half inch or so appears to be threaded. The one on my bike has a completly smooth bore. Can anyone tell me what those threads are all about?

Commando75 said:

when you look into the bore the first half inch or so appears to be threaded. The one on my bike has a completly smooth bore. Can anyone tell me what those threads are all about?

Click to expand...

Sounds as if it has been threaded for the 13mm reducing sleeve? (photo is RGM Motors 13mm sleeve kit)