?som includes a description of class 'som'.

component code 
 
a metrix with nrow = xdim*ydim, each row corresponding to a code vector of a cell in the map. The mapping from cell coordinate (x, y) to the row index in the code matrix is: rownumber = x + y * xdim

should say

rownumber = x + y * xdim + 1

because R unfortunately indexes arrays from 1, not from 0.

(and there is a spelling mistake in 'matrix' too)