question for repair techs

[Paul Scott]

I took one of my old Martins out to the backyard yesterday to give it a good cleaning. After pulling all the valve slides out I noticed what looked like a ring of grime where the slides would "end" in the valve tubing. When I looked more closely however, I saw what looked like a "tube within a tube" inside of the longer seemless valve tubes. The slides butt up against these "inner tubes" when pushed all of the way in. Perhaps this was an attempt to smooth out the bore keeping most of the diameter of the tubes the same as the slides. Or perhaps it was just to make them more sturdy (and sturdier horns are hard to imagine). I checked out several other older horns (King, York, Buescher) and did not see this feature. Even my youngest Martin (1957) was built this way as were my pre-War horns.
So, my question is: how did they do this? Is there a smaller tube brazed into the outer tube or were the tubes ground out at the ends somehow? I"m thinking that someone like Dan or Bloke who've worked on these beasts would have encountered this design element. I also wonder if any other manufacturers did this. Thanks in advance for any info.

[Paul Scott]

As Johnny Carson used to say, "I did not know that." Thanks for explaining that for me.