Its like I said above, a Brewer 42 was hybred with some combination of features and details normal to a Brewer 12.8 and a Whitby 42, without all of the features of either. It sounds like you have the underbody of a Brewer 12.8 with the ketch rig of a Whitby 42. You are right that some 42's had a sloop rig, all Brewer 12.8's were keel/centerboard cutters. I would am surprised that the mounted the oversized powered winches used on the 12.8's cutter rigs, for the smaller headsails of a ketch rigged version of the Brewer 42.
I have not heard of early Brewer 12.8's with blisters. I was a little disappointed to hear that your later Brewer 42 had a blister problem.
Its my understanding that there was a general upgrading of all of the systems on the original 12.8's, which well may have included electrical wiring as well. It would not surpise me if these systems were later downgraded on the 42's. That said, modern boats are generally designed for considerably more complex electrical systems than boats of that era and it may be that the system was undersized for the demands added to the boat after production.