This is the Caterpillar D9 which was first manufactured in 1956; and I believe is still in production (by Caterpillar that is). The Lesney model was made from 1960 to 1965. It was always yellow with a red engine. Variation comes from the different coloured rollers for the caterpillar tracks. They can be bare metal, metal painted red or plastic in yellow or red. The one to keep your eyes open for has the red plastic rollers.
It is usual for old Lesney caterpillar tractors of all types to have no tracks remaining, they were rubber and they perished over time, they were also fairly fragile and very prone to breaking in play.
