Markings On Stamp Backs

Hello,
Why do some stamps have symbols or just letters on the back of them?
Example: right bottom corner
I have another that has the letter ''H'' in blue Courier font.

Usually they are just previous owners. Sometimes they are from an expert in the field and I've accepted those as being genuine.

If a stamp is marked like this does it loses its value? Also can anyone ID this mark?

It has never bothered me adding a marked stamp to my collection if I wanted it. But that's just personal preference. It seems to have been prevalent many years ago. If you have ever seen Latvian pre-WW2 postcards, there was a collector by the name of O.Cinks who marked all of the postcards that passed thru his hands. That's a bit annoying but you get used to it after a while.

I would not want a marked stamp in my collection unless there was no other way to obtain such a stamp. But I have seen a number of stamps, especially older, more expensive stamps, with pencil marks on the back. Seeing such marks makes me cringe. I consider it the intentional infliction of damage.

I've had a number of stamps with blue stars on the back, like the example posted. No idea what the star means.

Some expertisers marked the back of stamps they looked at.
Here's a site with some marks. Yours may not be here, but it's always fun to look.

http://www.filatelia.fi/experts/index.html

Nice web site thanks!

The worst example of this I ever saw was on an R158A (current cat $12,500, but always rare), when someone wrote the entire description of it on the back, about 20 words. At least it was in soft pencil.
To quote Professor Moriarty to his butler in The Adventures of Sherlock Holmes, "You should be flogged, broken on the wheel, drawn and quartered, and boiled in oil".

ever check em for phosphors? pretty funn

also I imagine that some of this stuff is security measures . kinda like micro printing.
not every counterfit stamp ends up on a letter .
some are sold in sheets or rolls. so it could be that.

If a stamp is used or has no gum then writing or an owner's mark on the reverse, regardless of its affect on any given person's aesthetic sensibilities, does not affect the value of the stamp unless it is in ink that bleeds through to be visible from the front (this is a defect). Notations such as plating info from recognized authorities might even enhance the value. However, be aware that much incorrect information is also conveyed in this manner by owners who are shall we say less than authoritative.

Any markings on the reverse should be viewed with caution because the marking may be ownership, expertise or fake expertise ,then you have to understand both expertise items have been proven years later to be fake or a fake later with better research has been found to be real .
Once at a stamp auction I purchased a shoe box full of BATUM for thirty dollars as being fakes and used them for years as fillers in stamp lots that I sold ,sold a ton of those lots . About three years later a follow club member was over my home and he was a top Commonwealth expert . Fred Bell says to me that those stamps are both fakes and real stamps mixed in that half filled shoe box after I sold thousands as fakes. LESSON LEARNED ---do your own research .

Some stamps have back print originaly, like GB machin.
http://www.gbstamps.com/machins/faq3.html
Can that be the case?

Yes. And some stamps have descriptions of the issue on the back. Others have control numbers.

Backs of stamps.