I think the twisted logic goes something like this: Let's say you have an M2, an M3, the set of 3 LTM adapters and some lenses - a 50mm, a 90mm and a 28mm. For the 90mm, your choice is clear - you want the ISBOO adapter that brings up the correct 90m frameline on both cameras:
https://www.l-camera-forum.com/leica-wiki.en/index.php/ISBOO
A user with a more modern Leica like the M6 would also use the ISBOO with the 28mm lens, because it brings up the correct 28mm framelines (though it's not marked on the adapter, which pre-dates 28mm framelines). But you just have the older cameras, so showing the 28mm framelines isn't an option. So which of the three adapters do you choose? Really, you could use any, because you'll presumably be framing with an accessory viewfinder. But of course you'll also be using the rangefinder, which could make some choices more distracting than others, because some adapters will bring up extra, useless framelines. Using the ISBOO on the M2 or M3 only brings up the 90mm frameline, which makes no particular sense. So what does an M3 user do? Leica's engraving suggests the IRZOO:
https://www.l-camera-forum.com/leica-wiki.en/index.php/IRZOO
This is marked 'M2 50 M3 28-50'. It is of course the right adapter for the 50mm lens on either camera, but 'M3 28-50' suggests Leica also thought it was the best adapter for the 28mm lens on the M3. Why is this? Remember that the 50mm framelines are always displayed on the M3; you can't get rid of them with any adapter. Using the IRZOO at least doesn't add any
more framelines to distract you when using the rangefinder, and you are seeing the widest available framelines. How about using that 28mm lens on the M2? You could stick with the IRZOO, giving you the same single 50mm framelines. But Leica's engraving suggests you use the third adapter, ISOOZ:
https://www.l-camera-forum.com/leica-wiki.en/index.php/ISOOZ
This is marked 'M2 21-35 M3 135'. With the 28mm lens it will bring up the 35mm framelines (on the M2, there are no fixed framelines). Leica presumably thought this was less bad than showing the 50mm or 90mm framelines (35mm is closer to 28mm, after all, and you are again seeing the widest available framelines). My guess is that Leica recommended the 'least worst' option for each lens that was not supported by the framelines on each camera, which for the wideangles is the widest available set of framelines. Don't ask why there are no engraved recommendations for using the 135mm on the M2! (Perhaps they just thought that wasn't a recommended combination; you could use an accessory finder with the M2, but you'd be stuck with the lower magnification finder for focusing).