VIN engine code?

[1980]

I have a:
1980 k20 silverado-engine code M
1979 k15 GMC-engine code L.
What is the difference ? Both say 350cid 4B.

 

[pauly383]

Dipstick location , that and the 80 block is 4 bolt main .

http://www.cehighperformance.com/c_truck_block_codes.htm