These models are available either with riveted sides (left) or ribbed sides (right).
Sometime in the 1970s, Micro-Trains started releasing their freight cars (including this one) as snap-together / press-fit kits. These cost about a dollar less than the ready-to-run versions (no jewell box, though). Said kits were discontinued in the early 1990s after Micro-Trains decided that too many of them were being returned due to "missing" parts (conventional wisdom being that less than scrupulous modelers were buying the kits, liberating a part or two and then returning them for refund).
The purpose of a mechanical reefer is to keep perishable items cold. Early reefers were of all wood construction and used ice for cooling. By the 1940s, new reefers were being built entirely of steel. Insulating techniques improved to the point where economical refrigeration could be accomplished using steel side plates in place of wood sheathing. I'm told that these particular models are based on an FGE prototype first built in 1956.