Yep, it was a NUpowr 167 CPU daughterboard, Newer Technology - PowerPC 603e - 167MHz - 8MB RAM

gsteemso said:

You can find the complete set of Enablers in the Macintosh Garden. Not going to link it because I don’t remember if all of them were free on Apple’s discontinued FTP archive.

Click to expand...

I appreciate that thought, but that is literally the only one they don't have. The 550c enabler is literally not on the interwebs, I have looked everywhere, and the people I have contacted here that have a functional 550c have never gotten back to me, so it is indeed impossible to get anywhere. I've been wanting to dissect the difference between that enabler and the 500 series enabler for some time.