Replacement Crank Arm? Hi I have a 12 spline left crankarm to replace but can't find the right-hand side crankarm.

got ok for 12 spline its just the size.

My one is TR7JAED 165
All I can tell you is that its a 165mm long crank, and probably uses the ISIS interface to the bottom bracket.

Check your BB and see which one is most like your existing BB.  
 
I know of no 12 spline standard off hand, so either its a miscount of the 10 spline ISIS or you've got some weird standard.  Truvativ was known for weird oddball standards too... good luck!

It appears to date from the mid-late 2000s.

The pictured crankarm is a left hand side.  The right hand side crankarm has an integrated Spider to hold the crank arms, and looks something like this:

    Note this one has a square taper drive.  Yours has a different interface, so need to match the BB on your bike.

  
This one is a triple, because it has boltholes for more than one chainring.  It will be spaced so the middle chainring is in the middle of the cassette.  If you use this with a double chainring, it will sit a little too far out of line.

Other info: There's a wealth of measurements and data at https://www.sheldonbrown.com/bbsize.html  to help you find what you've got and what it is called.