Background
Both kits represent very late, post-1975 vehicles. This stands to reason, to an extent, because the only readily accessible surviving M51’s are from the final conversion batches. There are two Batch Four vehicles at Latrun (1999) and what is possibly a Batch Three vehicle at Har Adar (Radar Hill) near Jerusalem. Fort Knox has one, likely Batch Four, in storage, and there is another in Jacques Littlefield’s private museum in California. The only Batch One M51, to my knowledge, is at the Russian armor museum at Kubinka, which is not quite as accessible as the others. I believe that several M51’s and M50’s were captured by Egypt in 1973, so there may Batch Two vehicles there. Of course, over 100 Batch Four M51’s are just now being withdrawn from service in Chile. If there are other vehicles in private hands or on display elsewhere in Israel, I’m not aware of them.

What differentiates the various batches? For the most part, the changes affect the engine deck arrangement, air circulation around the engine and the exhaust.

Using the system previously published in several places:

Batch One: This is the earliest set of modifications for the Cummins engine. Because the Cummins went into the M50 first, then the M51, the modifications evolved. The most consistent feature is the engine deck, which has plain flat plates making up the rear-hinged portion. The exhaust exits from the lower rear plate, left side. The most common style of exhaust pipe is one that looks exactly like the left-hand pipe on the M4A3.

Batch Two: The engine deck was modified with louvers in the rear lid.

Batch Three: The rear portion of the deck was modified again to accommodate the exhaust, which is now diverted to exit from the center of the deck. An armored cowl protects it. As a result, a circular plate was bolted on to cover the unused lower exhaust port. A 52mm mortar was added to the turret top next to the cupola. A small box attached to the right side of the turret bustle was more likely used for storing a spotlight or similar non-explosive items, rather than the mortar rounds as is often stated.

Batch Four: There was still a need for additional air circulation, so the short-hull vehicles were drastically modified. The upper rear face of the hull was cut off and essentially tilted slightly back and up from the top edge, then welded back on with spacers to fill the gaps. This allowed for additional louvers underneath the overhang, thus allowing for more air to be drawn in by the twin radiator fans. These vehicles also have a large stowage bin added to the upper rear hull.

All short-hull Cummins vehicles also required a four-inch extension for the lower rear doors to allow clearance for the fan belts.

(Note: There is a pre-Batch One configuration, without the Cummins engine. See my book for photo evidence of M51’s in service with the original radial engine.)

Contrary to what some earlier sources have said, the two basic types of stowage layout seen on M51’s have nothing to do with these batch types. They are also not evolutionary in themselves, with one being older than the other. Instead, they were contemporaries, even being seen together in the same units from the mid-1960’s and later. In fact, each of the Batch Four M51’s at Latrun in 1999 has a different set.

Neither kit is a complete representation of any of these batches:

Tom Gannon

Other M51 articles:
DML Dragon M51 and back dating the kit
Academy M51 and backdating the kit
Making the ultimate M51