@lume: Probably, Mithril is said to be durable + light and of color silver (silverlight rapier *cough*) after all and Desir definitely felt it's far too light for an iron sword.
Maybe the inscription makes it so that it'll be identified as cheap iron sword to hide its true value, then future Pram eventually figured out the disguise, cry manly tears both at his own feelings and his father's hidden care, then swore to always use the rapier.
(Alternate option: The outside IS cheap iron sword, but there's actually an OP rapier hidden inside i.e. ch 5 of Handyman Saitou)
I'm guessing Desir will ends up jump starting his discovery via Analysis magic in a bit, similar to how he jump started Romantica's path to chantless magic.