I have noticed some similar markings on letters incoming to Amsterdam in the 1860's. Examples are shown below.
They are a series of numerals, some in black, some in blue. Is there a postal significance here? I also often see the numeral that looks like a '2' or '6' in red or blue on items arriving in Amsterdam. One item is from France, the other, Italy. I have others, but just found the file sizes to be too large, so will work with them later.
Suggestions as to where to look to figure this out would be appreciated.