Johan,
As Bujutsu pointed out, the bottom two cancels are segmented cork cancels. Very common, they don't add value but also don't detract value from your cover and card.
The top cancel from Portland, ME it is a large negative letter cancel. Portland used a variety of these cancels. Boston, MA used an even larger number of these cancels. There are several other US city's that also used this type of cancel. They are not cork but probably rubber. The US Cancellation Club has been doing a study of these cancels used in Boston.
I believe the use in Boston was to identify the clerk that cancelled the cover but did not relate to his name.
There is a US Postmaster site that identifies many postal clerk by name and town and date served.
http://about.usps.com/who-we-are/po.../welcome.htmI have a collection of postmaster's initials used to cancel US stamps.
I am the president of The US Cancellation Club and would be glad to send a recent issue of our Newsletter to any one interested. Just send me an email.
Vince Costello