The initial version of 1939 is called model number "A"
The superstructure consists of two parts, front and rear, that
are bolted together at a big flange on the inside of the open
fighting compartment just behind the driver's seat. The whole
of this hull is screwed to the chassis with clamps. The chassis
has armor on it too. Version A is intended as a test case for
further development. Most joints are welded. There is a report
of a version with mostly bolted joints. Some improvements are
incorporated in the "B" version that was made from 1939
till late 1940. This was only a stop-gap version. Some "B"'s
are reported to have some "C" Characteristics. A manual
shows a "B" sporting the "C" intake covers.
Mid 1940 sees the "C" version, which is the first version
that sees major changes to the design. At this point action is
undertaken to gear up production. Several new co-producers start
at this point. At first only the big contractors produce the "C".
(version /C showing the gradeability in the manual)
Very noticeable is a body-variant that is all bolted instead of almost all welded. The new participants in the production program are not equipped and lack knowledge on welding armored steel plates. They are allowed to produce the hulls with bolts. It is a specially adopted design. In this design there are other adoptions to this type of production. The top of the front bonnet is curved, not straight. The air-intakes are different to fit the bonnet sides. The two survivers (Bovington and Austin) have differences in riveting pattern. Problems with the very complicated production and an ever bigger demand lead to the final and most numerous version; "D". Largely simplified in design eliminating many angles and curves this design proved so effective that it stayed in production till the end of the war. The several designs to replace it are more sophisticated and more adequate in many aspects. But in 1944 even Guderian is in favor of unchanged production of the "simple" 251. Besides the German army, they are employed by the Rumanian army.
The A and B version have a distinctive different front section. Most noticeable by the front-plate made up out of 2 pieces. The left engine air-intake is operated with a lever by the driver. The A version has a very simple interior. See the drawings document. Clamps for the guns are on the top armor-sheet. In the crew compartment are armored visors. The radio is located above the right shoulder of the co-driver. The doors lock by pushing the lever flat to the door. Opening by lifting it. The antenna is on de lowest point of the right fender. The B version has no visors in the crew compartment. The lay-out of they equipment is slightly changed. An armored protection is added to the MG mounts. The radio is relocated to the front, facing the co-driver. The antenna is relocated to the top of the armor-plate directly after the co-drivers seat. There are B-version vehicles with changes officially new for the C-version.
The main reason: cooling problems. The two piece front-plate is replaced by a single sheet. The bumper is omitted. The air-intake is enlarged and gets boxlike covers. The frontal air intake is changed from a little hatch in a larger opening under the front-plate. The fenders are changed from the level ones of the A and B that are relatively near to the drive sprocket to a shape that has a bend at the point where the body-half's are bolted together. The headlights change shape. The turning-indicators are relocated from the driver and co-drivers visors to the fenders. The outside tools are relocated. Most noticeable, they disappear from the upper most armor-plate at the rear. The stowage boxes move to the back to accommodate the bent in the fenders. The front-wheels change in appearance (though that latter variant wheel sometimes is seen on the earlier versions). The seats get backrests which are lockers for weapons and equipment. The front-seats are changed to fold, and are attach to the floor-plates so the can be taken out with the floor for adjusting the driving brakes.
The air-intakes are omitted. The side visors are replaced by slits. The fenders are changed from the pressed slightly curved ones to straight ones. The stowage boxes are replaced by bins that close the entire space between the fenders and the armor plate. The bottom rear plate is extended to full height of the hull extending the hull. The doors are straight. They must be locked to stay closed. Simple bolts are seen as an safety guard against opening.
Photographed rarely, most known is a picture of a 251 /22 /D. The duel engine-cover is replaced by a single hatch hinged at the rear. The filler cab for the radiator is omitted, and the front piece of the bonnet-armor gets hinges on the right side (looking from the front). The interior is unchanged from C to D, with the exception of late production, where wood is sometimes used to replace metal in the seats and bulkheads.