The 2 3/4 in bp has female threads so you have to use an adapter to fit either a drill or the pc. I guess they are trying to make the smaller pad more versatile. Here is a pic from the kit.
2-3/4 Hook & Loop Backing Plate - LC-76-093
Our Price: $7.99
The 2 inch Hook & Loop Backing Plate is made of a solid piece of heavy duty, molded plastic with a thick pad of
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D.A Adapter 5/8" X 11 - LC-46-230DA
Our Price: $4.99
The D.A. Adapter 5/8" X 11 allows your dual action polisher to accept a rotary backing plate. The adapter works
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Spot Buffs Drill Adapter - LC-46-231
Our Price: $4.99
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3.5" Dual Action Flexible Backing Plate - LC-43-085DA
Our Price: $11.99
A molded urethane backing plate for excellent flexibility and balance. If German carmakers think this backing