'88 starter vs '98 starter

I have a 1988 xj 4.0 AW4. A few years back I installed a '98 4.0 starter in it, and it seems to work fine, although it sounds different than all the other starters in the XJ's that I've heard, and I was wondering if there are any differences such as a different gear reduction, number of teeth, etc. I looked through an OEM parts catalog and found that they had different part numbers, but then looked at the Shuck's web site and they were the same part number. Anyone know?

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