The 3B is the date of the card.

The letter represents the decade:

A=1930
B=1940
C=1950
D=1960

the number is the year within the decade...

3B is 1943

2A would be 1932

1C would be 1951

The code isn't always on the front of the card...sometimes it's on the back, sometimes in the stamp box. The code works for Curt Teich and C.T. cards, usually linens.

I forgot the Turf Club opened after the Northern closed, before the Monte Carlo Club...here's another 1943 card, with a view similar to yours

Here's a card date-coded 1951 (obviously a different location! )