I found on a letter dated 1904, Special Delivery, domestic mail to NYC a postmark, that I can not classify.
This is the front:
And here the back with the mentioned postmark:
This marked postmark is my problem. What kind is it? Does it have something to do with the special delivery? A postmark from the delivering mailman? Exist this only from NYC? Is there any publication, philatelic society or something, that research these postmarks? Exist a website about it? In the old publication "speedy" about these stamps I found nothing about it.
So many question because a small postmark. I hope some will be answered.