Umm, yesterday I sort of failed to notice, the cheap plastic ruler has some space before the mm scale begins. My bad...
The '51 cover height arch, measured over the string span, using the good ruler,
after coffee, is more like 21-21.5mm.
I also have a >57 cover kicking around. That's slightly more arched - I'm measuring 21-22mm.
Still sounds like a tight squeeze - what's up with that bridge?? Is there any way to lower those saddles a bit? (I wonder if the neck might be tilted back a bit too much.)
Alternately, you could shim up the cover. (Not sure how much clearance one could add before it looks bad.)
HTH