I recently ran into this, using a late model 1979 18V block, it was standard bore, we put the stripped block (including removing cam bearing) into his "hot" caustic vat for a good degreasing, and intial cleaning, then we started boring the block, this is to be 1900cc block (.080" over) we decided we go in two stage the first one being .040" then work from there to get to the final bore size, the cylinder took the .040 bore just lovely on all 4 bores, so now we went back to #1 and went for .030" cut, and thats where the trouble began. a well hidden from the top, a sleeve appear and start to spin with the boring bore, so we abandoned that cylinder, and got the block back off the boring stand and looked at it harder from the bottom for a sleeve. Ok the proper way to install a sleeve would be bore down the cylinder and leave a step on the bottom of the original clyinder to push the sleeve down to, thus leaving them no possible way to ever move, stepped to the bootom of the cylinder bore and captured on the top with the head, this is how almost every machine shop would do this job, however late in the 70s when BL was hurting for money , they pull blocks out of production, that failed quality control on bore size, that they use just throw away and now sent them out for sleeving, they speced the sleeves, they are normally way thinner than normal sleeves a machine shop was use. OK the crappy thing about how the factory puts sleeves in, is they just knock them and all the way down the cylinder and with no step and often a little sleeve hanging out the bottom, I've seen this on plenty of later 1275s, but this is the first MGB block I had ran into done this way, all 4 cylinder were sleeved the engine looked otherwise upopened before when I diassembled it, so I have no doubts this was a factory sleeved block.
Look at the bottom of your clyinder if you don't see where the sleeve was stepped into the original cylinder, and if you can see the sleeve coming all the way thru the cylinder to very bottom, then you'll know what you have, machine shop, or factory installed sleeves. I can say with little experience solidly proved what all factory sleeves are as for thickness, but we had to go well over .060" to distrub them, and even though I don't like the way the factory installed sleeves (stepless), they were still there after 3+ decades, so something has to be said for that. I would not be scared to build one of these block again, just not at 1900cc. I think I would easily trust the factory sleeves for a .040" over bore, and it all else you need to sleeve one cylinder your machinist can always sleeve the offending bore, I've had MGB racing blocks that got sleeved and then resleeve and worked out great.
AS for your question onthe exhaust valve cut out at the top of the bore, if a machine shop installed a sleeve into cylinder they would have to dress the exahsut valve notch back into the top of the bore, they would likely do this with a die grinder and mounted stone and cartridge rolls, just blending the old and new together, not a big deal.
Hope this helps.
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