Don't feel bad I accidently destroyed a pair of Dunstall liners. After I had them out and broken I saw how much they were worth. OOps..

Why not source a second hand barrell, the Atlas ones seem a bit cheaper than those for Commandos. I'm sure it would be cheaper than trying to salvage this one. I would certainly not be happy to leave those cracked lower sleeves even with a fresh re-sleeve job. As others have said you would probably be re-boring and fitting new pistons anyway.

You know, here is the funny thing. The barrels and top end are EXACTLY like the ones on my Commando. Are they not supposed to be?

I think they can be swapped.

I think the Atlas barrells have smaller diameter studs at the front and also a couple of the barrel base mounting holes are of different diameter, it's an easy fix to make them suitable for a Commando.

The bolts either side of the sparkplugs are smaller in an Atlas than any other Norton motor.
Not sure how long this continued for.
Makes them easy to pick against 500, 600, 650 and Commando parts.

Maybe this was only for when the cylinders had top spiggotts, and the heads machined for this ?

Well I took the jugs in for the Commando and the Atlas today. The Atlas had not been bored before, however they were cracked. I am getting an estimate for a guy to fix them. He is also going to bore and hone the Commando jugs at the same time. I'll let you know how it turns out.