First the predecessor to the FJ-40 was the FJ-25.  It was Toyota's interpretation of the old flat fender Jeeps.  I know Cruiser guys hate to admit it, but it is true.  They were built until the early 60's.  Then come the FJ-40's.
 

F Engine - 3878 cc's with 120 hp and 200 ft/lbs of torque.  These engines are best spotted by the metal valve cover gasket, side-of-block oil filler neck.  These engines were in FJ-40's until '74.

2F Engine - 4230 cc's with 135 hp and 210 ft/lbs of torque.  Easily spotted by the aluminum valve cover.  These engines appeared in '75 and went through the rest of the FJ-40 series.

3 speed transmission - through midyear of '74,  it was column shift in the early years, mainly before '72 allot have been converted over to floor shifters/

4 speed transmission - from late '74 up.  If it is a '75 or later it should have a 4 speed transmission

Smog Pump - This item starting being put on engines in late '72 and are still on Cruisers

Catalytic Converter - LandCruiser added it in the '79 year model

Drum Brakes - First baking system of the FJ-40's and went through the '75 year model.  They feature twin pistons which cause the trickiness in adjusting.

Disc Brakes - This was the biggest difference Toyota made in '76.  They were added to the front axle only and continued on.

PTO winch - I have only seen a PTO winch on 3 spd Cruisers which would hold them to the '74 year model.  Some claim to have them stock on there '75's.

Electric Winch - These were the replacement to the PTO winch, I believe and they appear to be Warn 8274 Models.

Air Conditioning - From '79 up as a factory option.  The factory unit is easiest to recognize by its ND symbol.  They also have a three groove crank pulley.

Power Steering - From '79 up as a factory option.  The steering columns on a P/S truck and non P/S truck are different by 1 1/8".

Radio - Appearing in the '74 year model, although only AM and located just above the heater box.

Spare Tire Carrier - '75 was the last year of this rattler, next was the tube carrier.

Round Taillights - Large round taillights until the 1973 FJ-40.

Square Taillights - '76 until the last FJ-40.

Round headlight Bezel - '78 down

Squared Headlight Bezel - '79 up

Hardtop Vent Windows - 1977 was the first year these appeared, the fold out for better ventilation.

Flip-up and Out Rear Doors - This three door combo went through 1974.

Rear Ambulance Doors - '75 up Cruiser had these on them.  They must be removed before the top comes off.

Cowl Vent - Allowed fresh air from outside at cowl inside, but prone to leaking, They discontinued using it after the '77 year model.

Bucket Seats - Added in '73, before there was a 40/60 bench.

Ox Blood Interior - The first FJ-40's featured this color.

Black Interior - In '68 Toyota went to this color.

Gray Interior - Only '79 ups have the darker gray interior.

Floor Mounted Emergency Brake - '79 up have the lever in the floor, before they were to the right of the steering wheel under the dash.

Steering Wheel - Round center on all before '81, then they went to a shield design.

Wheels - Steel with hubcaps before '81 after they used rounded steel white spoke wheels.