There has never been a Verizon, Sprint or AT&T specific factory image posted for the N6 by Google on their Factory Image page (
https://developers.google.com/android/nexus/images?hl=en).
There have been T-Mobile and Project Fi specific ones though. And for this, I'm not talking about OTAs. I don't care about those as I flash myself and haven't kept up on what OTAs exist.
So no, I do not think MRA58K, N or R are Verizon specific in any way. I've flashed all three of them right after they came out on my Amazon purchased VZE SIM using N6 and all three have worked fine. I did do a full factory reset after flashing MRA58K from Lollipopbut for MRA58N and MRA58R, I just line by line fastboot commands flashed with Minimal ADB and Fastboot from the XDA thread. Has always worked perfectly.
Seems as though Google posted N and then for some reason changed it enough to post R. Why I have no idea. A bit frustrating to have to flash multiple things, but certainly better than owning a never updated phone.