Composer's verification about Köchel number that is "5" (9)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

just a matter of getting the spelling right in this one. I’ve tried hard to like other stuff of Sergei’s apart from Romeo and Juliet but have never quite pulled it off. Oh! the parsing: K (Köchel number, AKA K, referencing the fellow who catalogued Mozart’s music) in PROOF (verification) IE (that is > id est) V (five)