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tell me since you've succeeded in this enterprise can I ask...

1) did you have issues the USB installer not booting because of 'blessing' issues?
2) was OCLP your Catalina install split into 2 partitions that each claimed file size equal to the media capacity?
 
No problems with 'blessing' - I've read that this is an issue but it wasn't with me. I downloaded High Sierra using the Patcher Tool with no problems. As it happens, I needed an earlier OS and I can't access it because of the Apple security issue.

I upgraded to Catalina from Mojave so had no issues with OCLP and it dowloaded as a single partition. I have a 21 inch Mac desktop 2019 so have no compatibility issues so far. I would guess that with the age of your three machines you do have, hence the necessity to find some workarounds.
 
Oh I see. That should have been a lot more straight forward I'd have thought. Surprised you even needed OCLP TBH.
 
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