The Adventure of the Blue Carbuncle - Facts

 

The hat that Peterson found is a billlycock. It is another name for a bowler hat. It is a hard felt hat with a rounded crown, originally created by the London hat-makers Thomas and William Bowler in 1849. The bowler, a protective and durable hat style, was popular with the British, Irish, and American working classes during the second half of the 19th century.

The street where Peterson sees Baker is Tottenham Court Road. 

The jewel which is found is a carbuncle. A carbuncle jewel is a deep red gemstone, traditionally a red garnet, that has been polished with a smooth, convex face.