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". . . It was during this time (1966-69) that Hamilton started to incorporate the highly innovative Buren Microtor (aka Micro Rotor/Microrotor) movement into certain upper tier watches (in addition to their ordinary hand-wind and traditional automatic watches).
The Buren (now Hamilton/Buren) Microtor was the disputed first automatic wristwatch movement to eliminate the sizable external oscillating weight inherent to most automatic winding watches. Instead, it utilized a much smaller weight that was entirely integrated into the chassis of the movement. This design allowed for a substantially slimmer automatic watch that still retained a center sweep second hand. The Microtor concept was also conceived by Universal Geneve for use in their famous Polerouter series of timepieces. The official title of "first Microtor movement" is still in dispute, in spite of remarkable similarities that exist between the Universal and Buren designs."
Source: Answers.com
The Buren (now Hamilton/Buren) Microtor was the disputed first automatic wristwatch movement to eliminate the sizable external oscillating weight inherent to most automatic winding watches. Instead, it utilized a much smaller weight that was entirely integrated into the chassis of the movement. This design allowed for a substantially slimmer automatic watch that still retained a center sweep second hand. The Microtor concept was also conceived by Universal Geneve for use in their famous Polerouter series of timepieces. The official title of "first Microtor movement" is still in dispute, in spite of remarkable similarities that exist between the Universal and Buren designs."

Source: Answers.com