There were 3 main series of 5 ton trucks used by the United States Military.
Here they are broken down by identifying characteristics and production years.

M39/G744 Series (1951-?)

• Air filter mounted externally on pass. side fender (Mack & Multifuel) or under hood (gas engine only)
• Flat grille
• Continental Gas Engine, Mack Diesel, or LDS-465 Multifuel
  

M809/G908 Series (1971-1983)

• Air filter mounted externally on driver's side fender
• Center of grille extended forward (similar to Deuce)
• Some have grille of steel tube
• All have Cummins NHC-250 Naturally aspirated 6cyl. Diesel
  

M939 Series (1983-?)

• Wider hood, flat and different styled grille
• No external air filter
• Allison automatic transmission
• Air brakes
• M939 and A1 series powered by same NHC 250 as the 809 series
• M939A2 powered by Cummins 6CTA 8.3 Litre diesel
  

Engines
Continental R6602 Gasoline Engine- M39 series
Mack ENDT-673 Diesel- M39A1
LDS-465-1A Turbocharged Mutifuel Engine- M39A2

Transmissions
M39 and A2 series with Gasoline engine or Multifuel engines- Spicer 6352 5 speed/manual (direct drive parent of the 3053A in the 2 1/2 ton trucks)
M39A1 with Mack - Spicer 6453 5 speed/manual (OD)

Most Common Variants
M51- Truck, Dump
M52- Truck, Tractor
M54- Truck, Cargo
M139- Truck, Bridge/Chassis
M543- Truck, Wrecker, Medium

Other Variants
M246- Truck, Tractor, Wrecker
M41- Truck, Cargo, with 14.00 "Super Single" tires

Engine
All M809 series trucks were powered by the naturally aspirated 6 cylinder Cummins NHC-250

Transmission
The Cummins was mated to the Spicer 6453 5 speed manual (OD).
As per the Spicer catalog, final drive ratio for the overdrive 6453 transmission is .78.
Variants of that transmission:
6453A .83 Overdrive
6453


Most Common Variants
M811- Truck, SEORLT, Gull Wing (Mobile Machine Shop)
M812- Truck, Rocket Launcher (?)
M813- Truck, Cargo, LWB (Long Wheelbase)
M814- Truck, Cargo XLWB
M815- Truck, Bolster
M816- Truck, Wrecker
M817- Truck, Dump
M818- Truck, Tractor SWB
M819- Truck, Tractor, 6x6 Wrecker
M821- Truck, Bridge Transporter (LWB)
"X" Prefix denotes pre-production run, as in XM813A1

Marine Corps Modifications/Variations
Marine trucks were outfitted with Detroit locking rear differentials. (To aid operation in sandy conditions, i.e. beach landings) Their air filters were equipped with extended breather stacks, and they were equipped with fording kits.
Some Marine M813s were outfitted with ISO beds, with stowable sides and accomodations to stow troop seats and canvas bows. They were designed to carry standard ISO containers.

Engine
Cummins NHC-250 Diesel- M939 and M939A1 series
Cummins 6CTA 8.3 Litre Turbocharged Diesel- M939A2 series trucks
-Interesting note- the 6CTA8.3 is smaller than the NHC250, rated at a slightly lower HP. It is identifiable by A) its single-cast head, B) it's turbocharger, and C) its smaller size

Transmission
All M939 series trucks were equipped with an Allison automatic transmission

Transfer Case
High Range: .732, Rated Input Torque- 7350 Foot Pounds
Low Range: 1.79, Rated input Torque- 650 Foot Pounds
Oil Capacity: 13.5 Pints
Weight: 460 Pounds

Most Common Variants
M923- Truck, Cargo
M925- Truck, Cargo, with winch (hydraulically powered)
M931- Truck, Tractor
M936- Truck, Wrecker
M945- Truck, Chassis (Bridge Truck)

Some Marine M923s were outfitted with ISO beds same as their M813 trucks

Production Note: The M939 series was originally designed as an upgrade package for the M809 series trucks. They only began to be produced from "scratch" when the M939A2 series began to be manufactured

The M939 series and the M809 series trucks are being replaced by the FMTV (Family of Medium Tactical Vehicles) in the Army, and the Oshkosh MTVR/MTVR-II series (Medium Tactical Vehicle Replacement) in the Navy and Marine Corps.