Baseplates for the installation on the bottom of a standard dog ear P90. These are standard sized and will fit any dog ear P90 of 50mm pole spacing (the length from the centres of the outermost pole screws).
Nickel silver is commonly regarded as being superior to brass due to the fact that it creates less capacitance in the coil of the pickup. Any metal in close proximity to a pickup coil can cause capacitance, which attenuates treble (think Telecaster neck pickups for a very clear example of this). Brass does this to a more significant degree than nickel silver, and for that reason, most people prefer nickel silver to preserve their treble as much as possible.