very nice photos. I always call them "makers" marks, i.e. like Ron said, parts was made, and checked by someone (could be an inspector, could be a foreman or lead worker in that area).
somethings we'll just never know for sure
Also, although there "could" ber rhyme or reason for some markings i.e. the #3 or #5, mostly likely not if they aren't the last two of the serial number....