The joke here, though, is that Stradivarius violins are VERY rare and VERY valuable.
So although "Seen one, seen 'em all" is a very common thing to say, in this case, this expression doesn't apply at all - which is where the joke comes in.
When you've seen one X, you've seen them all. = All X's are the same. There's nothing to be impressed by if you've seen one. This expression is similar to another saying: "They're a dime a dozen".
The joke is that Stradivarius violins are rare and expensive, certainly nothing any knowledgeable person would call unimpressive. It illustrates just how comically clueless Ins
Oh I see. The way he said the phrase was so pretentious that I thought, "there must be some classic original phrase." Now I understand the joke, and I'm so grateful to you all. "They're a dime a dozen," huh...