While I could look at my Firepower, I don't think I need to do so.

All of the switches you refer to are, according to the switch matrix table, meant to be connected by a WHT-VIO wire. (Slam Tilt, Spinner, "6" Target, Upper Top Right Standup, Top "Power" Target, Playfield Tilt - see http://mirror2.ipdb.org/files/856/Williams_1980_Firepower_Instruction_Booklet.pdf.)

So what I am guessing happened in the past was that there was some fault in the WHT-VIO wire on your machine (a short, an open, melted onto a coil, who knows) and the owner/operator decided it would be better to simply run a new wire. But s/he did not have any WHT-VIO wire so s/he used the nearest thing at hand being some solid purple wire.

So, IF I am correct, it is not a hack but a repair. If it were me, I would certainly replace the purple wire, with some WHT-VIO wire. Depending on whether I could be bothered, I might retest the existing wire on the playfield to try to work out the original problem. (I would probably be too curious to pass up the opportunity to waste my own time.) More efficient, in terms of time, would be simply to get some new WHT-VIO wire and run the string again AND remove as much of the old wire as is easy.