cal. 51111

hello, i am new here and recently considering a 500901 or 500420 as my first entry into the bp series. both, if i remember correctly, have the cal. 51111 in it.
with the price difference and the latter being a limited production, is there any difference in the materials used or level of finish in the movements? is there no inner soft-iron shield within the 500420? if so, how that matters in its anti-magnetism?
thanks in advance.