If the part in question ("step plate" or "strips that are used for the door seal") is the "lower windlace retainer", here are some photos that give a little idea of differences during Advance-Design years:

1947 to mid-year 1949 - stainless steel

1947 to mid-year 1949 - steel

mid 1949 to 1955-1st - stainless steel

mid 1949 to 1955-1st - steel

I am assuming that the lower retainer changed when the style of door windlace channel guides changed sometime in 1949? (Classic Parts does not give the impression that the lower retainer changed when the door-surround-guides changed).