YAMAHA Compact Small Mass Bridge Introduced in 1982 for SG and SA Guitars
In 1982 Yamaha introduced the SG 3000 which had a number of new features, including the Spinex pickups. One other new component was the 'New Small-Mass Bridge'. This bridge is both smaller and lighter than its predecessor which was used on the SG and SA of this era.
Here's how Yamaha described this bridge:
State-of-the-art bridge design in the SBG 3000 results in an unconventionally compact, lightweight bridge that efficiently transmits the string vibrations to the body of the guitar, creating exceptionally live string response.
While exclusive to the SG 3000 at introduction, it was later used across the SG and SA lines.
I've compared a number of the two style bridges on both SG and SA models. I found that the small mass bridge is 0.6 ounces/17 grams lighter (2.6 vs 2.0 ounces / 73.7 vs 56.7 grams) than the older style bridge.
Here are some photos of the two versions.
Yamaha SG and SA Compact Small-Mass Bridge vs Large Bridge
The Compact Small-Mass Bridge was designed to be more responsive, but has less adjustment for intonation.