Tom: I noticed that too.

Casper:
Bit out-of-focus but muzzle photo in your previous post looks a lot like the rifled barrel has a full-length sleeve.
What looks like the join is visible at the muzzle. In the UK it would be Parker-Rifled.

No suggestion of it having been a SxS shotgun as the left-barrel wall thickness is greater than the right, even without the sleeve.

If it simply has an engraved circle on the face of the rifle barrel's muzzle, then disregard the above!