Since I've had several requests, I've decided to post a list of Florida county code numbers. Florida license plates used a number code to represent the county the plate was issued in from 1938-1975, with some of the 75 bases being used up until 1980 by being renewed with stickers.

Anatomy of a 1938 - 1975 Florida license plate:  1w - 23456

1 = county code (DADE county in this case)
w = weight code
23456 = serial

There is an exception to this rule though, FL REAL PROPERTY (mobile home) license plates are backwards from the regular plates: 1234RP5
1234 = serial
RP = real property
5 = county code (POLK county in this case).

IMPORTANT NOTE: PLATES WITH THE 68 NUMBER CODE ARE NOT GOVERNMENT PLATES LIKE I HAVE SEEN THEM ADVERTISED. THEY WERE ISSUED TO PEOPLE WHO RENEWED THEIR PLATES BY MAIL WITH THE MAIN DMV OFFICE IN TALLAHASSEE. THIS ALSO APPLIES TO NEWER GRAPHIC FL PLATES THAT SAY DMV AT THE BOTTOM. A real DMV government owned vehicle plate is an undated black on yellow plate with the format DMV 123.

That being said, here is the list:
1      Dade               24      St. Lucie          47      Citrus
2     Duval               25     Jackson             48     Clay
3     Hillsborough     26     Osceola            49     Hendry
4     Pinellas             27     Highlands         50     Washington
5     Polk                 28     Pasco              51     Holmes
6     Palm Beach      29     Columbia         52     Baker
7     Orange             30     Hardee            53     Charlotte
8     Volusia             31     Suwanee         54     Dixie
9     Escambia          32     Indian River    55     Gilchrist
10     Broward         33     Santa Rosa     56     Hamilton
11     Alachua         34     DeSoto           57     Okeechobee
12     Lake             35     Madison          58     Calhoun
13     Leon             36     Walton            59     Franklin
14     Marion          37     Taylor             60     Glades
15     Manatee        38     Monroe          61     Flagler
16     Sarasota        39     Levy              62     Lafayette
17     Seminole       40     Hernando       63     Union
18     Lee               41     Nassau           64     Collier
19     Brevard         42     Martin           65     Wakulla
20     St. Johns       43     Okaloosa       66     Gulf
21     Gadsden       44     Sumter           67     Liberty
22     Putnam         45     Bradford        68     DMV Issue
23     Bay              46     Jefferson         90    Replacement
Hope you find this information helpful :-)