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Do a search on the subject there is at least 50 threads on this question.
There is no significant difference in the AP2 rear diff. The reason they are less likely to break is because of a clutch delay valve used in the slave cylinder of the AP2 transmission.
I have had to repair or throw away many but, not as many as the AP1.
The other change made was the use of a different style bearing cap, this bearing cap is only new to the S2000 in 2004 plus S2000 cars. The cap design has been used for Mazda in the 80s and 90s.
There is no added strength, it just looks like there is, the radius is still the same thickness where it is subject to break.
The AP2 rear you got is a good rear, use it. The gear ratio is the same (4.10) as the AP1 the only issue you will have installing it is that the input flange uses a larger 10 mm socket head screw. You will have to change out the input flange to an AP1 style flange that uses a 8 mm fastener.
Please understand that most diff failures in the car is due to driver error not mechanical failure. Be mindful of what you are doing and the rear will last for hundreds of thousands of miles.
