M4 rods w/suffix J are equivalent of early no-letter rods.
M4 rods w/suffix K are single taper and step to .026".
M4 rods w/suffix L are equivalent of 4M "B" rods.
M4 rods w/suffix M are single taper and step to .036".
M4 rods w/suffix P are single taper (steeper than K rods) and step to .026".
M4 rods w/suffix S are smooth single taper to .036".