On most 1980s Alembics that is exactly where you would find the serial number.
The soldered pickup wires are a bit odd, but of course there may have been a problem with the original connectors, and they may have just solved the problem this way. Other than that, you can usually recognize the original Alembic electronics, even if they're not stamped - why don't you post a picture of the insides of the electronics cavity?
Weight-wise, you must be comparing the Spoiler 5 and the Persuader 6? If they have the same body style, and the same scale length, there should be some difference because of the wider neck on the 6 (maple can be pretty heavy) - afaik the wings are the same size no matter how many strings you have. The extra couple of ounces for the 6th tuner shouldn't be too noticeable. And some of the difference may be to do with different wood options.