That looks like an occlusion in the steel that may have been there forever.
I have a couple of forged standard 3.31 stroke cranks that need to be cleaned up....a polish may be all they need. Not 340 cranks but I wouldn't care if it was me. $75.00 to you and I might be able to deliver 1/2 way.
Heck make a welded stroker out of one. Or I'll sell you a 360 crank and grind the mains to fit that 340.
