SM, SM II etc are waterproof versions of the old Whites Classic VLF. ie not Pulse so better discrimination but much less depth. Best kept for freshwater use though can be used on land.
Surfmaster PI's must have hit the market about 1991. Original models could take an extra couple of batteries so that the rechargables of the era could be used without loss of performance.
Then Surfmaster P.I. that couldn't take the extra batteries but batteries had improved so not a real problem.
Think the Plus was next with improved sensitivity to small gold.
Then the P.I.Pro with its 'all new power circuit'.
If it says Beach its got a lighter coil, Pro Diver or Deep Diver has a weighted coil and a short divers rod included in the package.
The P.I.'s always cost more than the none P.I.'s and older models of the P.I.'s are not as good so cheaper. Check the date of the detector.
Then remember that Bill Crabtree would convert older models to a higher spec than the newer. So check if its a Crabtree special.
Latest version in the shops is best of all and no worry about any possible water leaks in the past that might be corroding the circuit board away.
