When Jos Tufton asks Mrs. Patmore for the honour of "squiring" her around at the fair, she marvels that nobody has wanted to squire her since the Golden Jubilee. This refers to Queen Victoria's half-century anniversary on the throne, which occurred in the summer of 1887 almost 35 years before the events in this episode.
Violet says that she wore "the crinoline, the bustle, and the leg-o-mutton sleeve." These were late 1800 fashions. The crinoline was a hoop skirt that later morphed down to hoops just in the back (the bustle). The leg-o-mutton were sleeves with much fullness at the shoulder and tapered to the wrist.
While hunting with Shrimpie, Robert mentions the Marlborough's divorce. He is referring to the divorce of Charles Spencer-Churchill, 9th Duke of Marlborough, and Consuelo Spencer-Churchill, Duchess of Marlborough, whose separation in 1906 and divorce in 1921 shocked British society.
When Lady Rose comes downstairs for dinner, she's scolded by her mother for wearing a too revealing dress. She answers her back that Princess Mary (King George V's daughter) has one just like this. Considering the King's conservative taste and the fact that he would not allow his wife to follow post-war fashion trends, it seems rather unlikely that he would have let his unmarried daughter wear such a dress.