This week I had the same with an XT mid ring on an XT crank. The crank was modern (external bearing BB). The original ring was what I would call a usual XT steel ring. The replacement was a newer XT ring, chrome steel with a wadge of molded black plastic (polyvinylshiftate?) with alot of the shifting ramps molded into it. The internal diameter of the mounting points of the old/new rings were different (but bolts holds were the same), the new ring could only be fitted if one were to file/grind 2 - 3mm off the inner part of the chain ring.
The old ring on top (square corners), new one underneath (curved around the bolt hole that fouls the crank arm shoulder). The bolt holes do line up, even though they are a bit cock eyed in the photo.
How long the plastic shifting ramps will last is a mute point.
