Well, In the early days there were "A" series engines, small two cycle engines produced by RPM. Then we saw the birth of the "C" series engine and the foundation to the ever popular "D" series engine produced in the sixties and seventies with engines such as the D-400 and the later electronic ignition equipped D-600 in 1975. In 1976-77 Lawnboy produced a fabulous little engine known as the "F" series and was mass produced till O.M.C's demise in 1990ish and sold to the Toro company.
Lawnboy had designed the "M" series engine and deck but Toro continued to sell it as well as the "F" series engine up till around 1997-8. In the mid nineties Toro produced under the lawnboy name the "V" series engine which was a budget series project with a few quirks. Sometime in the late nineties Toro produced the "E" series or the dubbed Duraforce engine and was stopped of all production by the epa because the large two stroke engine could not pass emmissions regulations.

The factories of the old O.M.C plants moved around a bit from Plymoth, WI to Lincoln, NE and Galesburg.
Toro has produced and built all their power equip in Minneapolis, MN as far as I know. But the new stuff is anyones guess now, like you said probably Mexico.

"Could someone give me a brief summary of how to know what is what."

This could get a little long. The old "A" had little or no cooling shroud.

The "C" was more modern looking, had a recoil that pulled sideways like a typical recoil. Some parts from the last versions of this engine would interchange with the "D400".

On the "D400 and D600" both, the "2 finger" start was introduced and the recoil pull directly up at reduced speed. The cylinder jugs of these engines had a re-inforcing web between the transition from cylinder cooling fins to head cooling fins. On the "D600" the web on one side forms a letter "H" in the web. This stands for high compression. All of these engines bolted flat on the mower deck with 3 bolts. Crankcase is one pc. with magneto plate bolting to the top. These and the "C" had a model number tag riveted to a head cooling fin. While the jugs from "D400 and D600" will physically interchange, don't use a "D600" jug on a "D400" as the flywheel does not have enough cooling fins and overheating will likely occur.

The "F" engine went back to a side pull recoil. When looking at the carb side, the engine looks like it is standing on two spraddle legs, like a Sumo wrestler. Also bolts at the exports. The crankcase splits thru the crankshaft. Engine stands above the muffler/mounting plate.

The "V" looks much the same as the "F" except the cylinder cooling fins are larger and form a square looking jug. Horse power was increased along with compression thus, more cooling fin area. This and previous engines had the muffler and exhaust below the mower deck where they should be.

The "E" or DuraForce is the first with removable head and went up to 6.5 hp. Muffler is above deck.

I am not familiar with the "M" except it had separate oil and gas tanks injecting oil into mix as it ran.

I am sure others will have better explanations.

Walt Conner

Wow! Thanks guys for all the info!