I dont know where that coupling came from but it doesn’t look like any I’ve seen either. you should set it aside and just order another. You could also upgrade the Z by building a linear rail version but just a new couling should be enough to get started.
As for the upgraded ring, you can either go with a ring kit or linkage kit that @dlang referenced.
If this kit was one of the ORIGINAL @Bar kits then the remaining motors and electronics should be just fine for continued use.
I will mention that the earlier left / right motors used an un-hardened drive gear in the gearbox that was prone to early wear so you may have to replace the gears or the whole motor assembly eventualy.
Asside for the Ring or Linkage Improvements, the main improvements to Maslow seem to be in the firmware and various control software solutions. Upgrading to Web Control and Holey calibration will make a huge improvement too.
Simply put, get a new coupling and start assembling/testing even with the old L bracket hardware just get the project moving forward. Upgrade to a ring system or linkage kit as soon as possible. Use WebControl w/ Holey calibration from the start.