BBC Crankshaft I.D. Help

Can anyone here help me? I got a crank from a buddy and he said it's an Eagle or Lunati BBC 4.25" stroke 4340 crank. I have no reason to doubt him & he has no reason to lie, but I would like to know for sure.

The only markings on this crank are on the front journal. It reads:

M-12 256

Can anyone tell me what I have? I have googled it in every variation and came up with nothing.

Thanks!

 

put it in a block (on bearings), and measure the stroke?

Call Eagle and Lunati and ask them how they mark their cranks?

 

Big Block Chrysler?

 

Big Block Chevy

 

I knew that, I just hate when Chevy guys think the C always means Chevy