Either the P6x7 or P67 metered finders (branded Asahi Pentax, Honeywell Pentax or just Pentax) will fit and work. The later one for the P67ii will not fit or work.
The procedure is to remove the lens before replacing the finder, replacing the lens after the finder. You risk breaking the metering chain if you replace the TTL finder with a lens mounted.
If you buy the metering prism finder make sure you get the round plastic part which slips over the body's shutter speed dial.
This piece couples the meter to the body and is occasionally missing in online auctions.