No, well not really. I believe it was about 1.5 - 2 years ago, we compromised a bit with RAAL to make manufacturing of our custom S2 tweeter and the RAAL OEM 64-10 easier by having them share more parts. No differences in performance, just a very slight change in overall size of the components on the back side of the tweeter - such that newer S2/S2EX tweeters will require a little bit of cabinet filing to slightly open the internal cutout of the tweeter to fit it in older cabinets (a few mm's at most). For anyone doing a Sierra-1 to Sierra-2EX upgrade, or ordering replacement S2 tweeters, we provide the proper S2 tweeter so that filing isn't required. That is at least until we run out of our supply of the S2 tweets.
For the most part, this revision is invisible to our customers. I had previously discussed this in the forum somewhere. This helped RAAL lower manufacturing costs and in turn, helped us keep pricing the same, otherwise we were looking at an increase in price.
This has nothing to do with why we ask for serial numbers, we request this for all upgrades so that we can properly track the speakers. We have had issues in the past with people selling used speakers claiming they were upgraded, but were actually never upgraded...