What are the plates suppose to accomplish? Shield?Assuming you're talking about a steel add-on plate...
They're all about the same. The thicknesses will vary a bit between manufacturers, but the average will be 3/64" which is 0.046" There might be a change in their effect with a significant change in thickness, but who knows. I'm going to cut base plates from 3/16" thick bar stock and try them under a set of Tex-Mex Strat pickups, just because.
Stick them on with double-stick tape or hot wax. The goals are to keep them on and have no large air gap between plate and pickup base: we want nothing that might resonate. For god's sake don't glue them on. It's unnecessary and you risk destroying the pickup if you want to take them off.
Yes, they cost actual money. Folks think 'it's just a piece of metal with holes in it.' The costs of sheet metal, tooling to punch it, tooling to drill and tap it, hand finishing, labor, etc. add up.
What are the plates suppose to accomplish? Shield?
The original intent was likely to provide shielding,
That was my guess.
Combined with the bridge base plate and the neck pickup cover on a Tele, it sort of makes sense, thematically.
That bridge assembly design was originally used in their lap steels, and then adapted to the guitar.
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It's interesting that for the Strat, Fender decided to do away with shielding all together, and not use three Tele neck style pickups for the Strat's three pickups. They must have figured that shielding didn't make much of a difference, which compared to humbucking, it doesn't.