Sometimes a letter appears after the PLAT indicates a code for the alloy - in other words standard 950 parts platinum and then the alloy L makes up the rest. I said code for the alloy because it doesn't necessarily stand for anything that is universally understood, just a specific to the jeweler.
Of course it could also be the size of the ring - would be an odd thing to put on there but people do odd things sometimes. L translates to 16.40mm as a size.
Anything platinum plated would not require a hallmark (speaking from the UK here though).
PLAT means its platinum .950 as far as I am concerned.
Where did you find your ring - if it was reasonably deep and you can assume its been down there awhile then the dead giveaway is that platinum, like gold doesn't tarnish - it comes out looking as good as the day it was lost.