The Boeing 737 is the most sold aircraft in history.
The 737 consists of 4 generations, being:
(1) The original: Boeing 737-100/200/200(Q)C/200F
(2) The classic: Boeing 737-300/300(Q)C/300SF/400/400(Q)C/400SF/500
(3) The next generation (NG): Boeing 737-600/700/700C/700SF/800/800BCF/900/900ER
(4) The MAX: Boeing 737MAX-7/MAX-8/MAX8-200/MAX-9/MAX-10

The 737-300 is the most popular variant of the 737-classic, it was also the first variant of the classics. The aircraft entered commercial service in 1984 and is still in use today.
The 737-300SF was not offered by Boeing, however, the aircraft is a converted version of the 737-300 or a 737-300(Q)C, which is permanently configured in a cargo configuration.
The 737-300SF is relatively popular with cargo operators, operating on shorter routes.

I have also made the other aircraft from the 737 family.
737-Original:

737-100

737-200

737-200(Q)C

737-200F

737-Classic:

737-300

737-300(Q)C

737-400

737-400(Q)C

737-400SF

737-500

737-NG:

737-600

737-700

737-700C

737-700F

737-800

737-800C

737-800BCF

737-900

737-900ER

737-900ER (With Type II Exit)

E-7 Wedgetail

P-8 Poseidon

737MAX:

737MAX-7

737MAX-8

737MAX-8-200

737MAX-9

737MAX-9 (With Type II Exit)

737MAX-10

Controls:
AG2 + Brake for reverse

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