Possible set dressing reuse, Beau Geste / TOS. Maaaaaybe.

[Treadwell]

I spotted a plaque with knives in Beau Geste (1939) and thought it might have been used in "Mirror, Mirror". But after noting that it's a different color, and different knives, and the round things relocated, I dubbed it "close but no cigar." But the Star Trek Prop Enthusiasts group on fb thinks it's still possible it's the same, just reworked. The movie was a Paramount production, but Desilu was bought by them during the filming of this very episode, so it's unlikely the production yet had access to Paramount stores of props etc.... unless there was a casual "hey, you don't own us yet, but you will soon, can we just come over and check out some stuff?" "Yeah, okay."

Using Mythbusters standards, I mark it as "plausible".

 

[BLASTER]

Interesting …The plaque and the four outer knives look to be the same ones in both stills just that the two outer knives in Star Trek look sheathed. The center knife replaced with another.
Can’t go by color as Star Trek loves the colored lights.
The display could be a prop house rental…. Probably still sitting on a shelf somewhere…

 

[CessnaDriver]

Seems so close. The two round features on top do not line up, and there are three layers instead of two.

 

[joberg]

Yep, I'll agree with a "close, but no cigar" on that one prop

 

[Stairstars]

I think it is a safe bet that the display plaque came from Stembridge Gun Rentals, then housed on the Paramount lot, as they had numerous such pieces decorating their space. This image from a 1969 magazine article on them: