John Steed Fashion

A perfect gentleman is fixing his tie with a tie pin, and it can save a gentleman's life.
Steed's diamond pin which he bought at a jeweler in Bond Street cost "seven hundred and fifty guineas." That's what he told Jessop in Esprit de Corps. Steed bribed the execution squat with the tie pin, and after the execution, his tie pin belongs to private Jossip.

A tragic loss, but obviously he has a few more, and he gets one as a present from Mrs. Peel before the episode Two's a Crowd. It's a special gift because hidden inside is a miniature camera, and Brodny knows that Steed has worn it many times.

Later Steed's double, Webster (who is Steed himself), is wondering when Steed/he got the tie pin: Christmas, birthday or some special occasion - we will never know, but as Too Many Christmas Trees was produced before Two's a Crowd...More interesting even is that he already used that tie pin in The Murder Market.
Dial a Deadly Number allows a good look at the tie pin when Steed is dressing
But not only the pie pin can be seen, have a look at his flat keys, the wallet and his Hunter watch too.
Steed wears most of the time that kind of tie pin, but there are other models too, one with a small pearl or the golden one with the ruby from My Wildest Dream or Look (- Stop me if you have heart this one) But There Were These Two Fellers...

There are different looking tie pins:

1. The golden knot tie pin from Two's a Crowd
2. The yellow gold and diamond tie pin from Esprit de Corps
3 The diamond tie pin as seen in many episodes such as Dial a Deadly Number
4. The ruby tie pin or tie tack as worn in series 6 episodes such as My Wildest Dream
5. A brilliant tie pin from Legacy of Death
6. A pearl pin in The Correct Way to Kill (besides the cream pearl pin Steed also wears a light blue one)
7 A floral tie pin in The Correct Way to Kill.