County codes Weight/Letter codes do not matter to DMV
check with DMV to verify a number is "clear" in the system - you may use this link as a quick check, but a call to DMV is better:
The following section is from DMV Manual RS-25 Rev. 10-07-2020 (downloads as .pdf)
IV. REQUIREMENTS FOR AUTHENTICATION OF A LICENSE PLATE:
Authenticated license plates may be issued to any vehicle manufactured in 1975 or earlier. The license plate must be the same year as the model year of the car or truck. The license plate will be approved for issuance by the department as an authenticated license plate if the license plate has not been refurbished in any way and is still legible for law enforcement purposes. Authenticated license plates may only be issued to cars, motorcycles, and pickups weighing 5,000 pounds or less.
The documentation listed below must be submitted by the applicant to the following address for authentication:
Department of Highway Safety and Motor Vehicles
Direct Mail and Issuance Rm #A330
2900 Apalachee Parkway, MS #72
Tallahassee, FL 32399
A. A letter requesting the authentication of the license plate. The letter must include the following information:
1. Mailing address (printed). If an out-of-state address or a post office box is used, a Florida residence address must also be provided unless the applicant is an active duty member of the Armed Forces of the United States who is also a Florida resident. Such an applicant is exempt from this requirement.
2. Driver License number or owner’s sex (using “F” for female or “M” for male) and date of birth (month, day, and year). If joint ownership, give the information for both owners.
3. A description of the license plate (the year and license plate number).
4. A description of vehicle (the year, make, VIN number, and title number).
B. The license plate (for authentication purposes) furnished by the owner. See Exhibit C for correct year and colors of license plate.
C. Payment of the applicable fees. NOTE: The Initial Registration fee is not required on vehicles 30 years and older.
D. Proof of Florida insurance. Refer to RS-36 for insurance requirements.