1986 350 Vin Code L or M

Anybody know what the difference in the Vin Code L or M on a Chevy or Gmc with a 350?

Does it mean Heavy Duty or Light Duty vehicle service for usage/cam/cooling/transmission, etc...?
Or is it, Califorina Emmissions or Federal Emissions?

I have heard it as CA Emmisions or FED, but not sure if they also mean Light Duty Vehicle Service or Heavy Duty.

had the same concern when i swapped a 305 for 350 in my 86 c-10. main difference is the bolt hole size is different on the heads. insure that main seals are the same. one may be one piece and the other two. oil pans are different.

On a 1983 vin the M is for H.D. "heavy duty". I think all c/k 20,s and 30's have M.