Spots on the neck

No, the title does not refer to some illness or state of mind, but to matters that are important to collectors of plate errors on stamps. The Special Catalogue of the NVPH includes a section titled 'Plate Errors'. Not only plate errors on Dutch stamps, but also those of the former colonies. Including those of the Dutch East Indies. While searching through a box of mail from the Dutch East Indies, I came across the first (and only) issued first-day cover, dedicated to the 50th anniversary of Queen Wilhelmina's reign. The envelope was affixed with two 15-cent orange stamps and a 20-cent blue stamp, and cancelled with the first-day stamp of August 31, 1948. Additionally, there was an impression of the Batavia-Centrum 2 date stamp in purple-violet with the same date. The envelope was addressed to Alkmaar and was registered. A Procisor registration slip was used, on which the name of the Batavia-C office was imprinted with a small stamp. So far, nothing unusual. Until I started examining the stamps a bit more closely and noticed that on the top 15-cent stamp, something appeared on the other stamp that was not there, namely a sort of code or letters in the form of cl. I grabbed the catalogue and sure enough, I discovered: the 'plate error' was neatly noted as number 337P with the caption 'orange spots in the neck of stamp 10'. I decided to keep that envelope after all! Was that all there was to see on the envelope? No, because written in pencil at the bottom right was also: *zoz*. Was there a remark about the spots there? I turned the envelope over and, lo and behold, the addressee was not at home when the postman came to deliver it. He wrote: notice in the mailbox 9-9-'48 D. Now we also know immediately that the journey had taken nine days. Do you ever look at the stamps to discover plate errors?