Sure it's sleeved?
I've a Greener which shows a subtle change in pattern near the breech, I believe from being a different, thicker section of tubing when the barrels were first forged - not sleeving later.
I've a John Manton 15b which shows a similar thing; you can see the spot in the pattern where the different sections were welded together.
That siad, I guess if a decent set of damascus tubes were available, it could be done.
RG