Marking Stamps With Pencil

I have acquired several stamp collections over the years. I noticed that some stamps are marked with catalog number on back of stamp (primarily used). What is the protocol for this? Yes? Used? Unused? Thanks.

It happens. Normally, with a soft pencil. I am not a big fan of it when it comes to unmounted mint stamps. Going over original gum with an eraser is not something that improves the gum. For unused stamps, erasing it, likely, leaves less of a trace than the hinge. For used stamps why fuzz about it when you can erase it?

Pen markings and markings with hard pencils are more of a problem, as they might not be erased (completely) or leave a recess.

The other problem is that the marking may give an incorrect Scott/SG/whatever number to add to someone's confusion later on.

Very common and generally looked on favorably when the pencil markings identify a [correct] position on a used platable stamp. I think it is a bad idea to mark a mint stamp or just to identify the catalog number.

Welcome to the forum, arbob!

A pencil mark on the back of a stamp is part of its history, whether it's helpful or misleading, so I'd be reluctant to try to erase it. It isn't going to cause increasing damage with the passage of time, while attempts to erase it may be ineffective at best, disastrous at worst. And perhaps one day it will be authenticated as the handwriting of Franklin D Roosevelt or John Lennon!

If an annotation on the back of a stamp is erroneous, I think you'd be justified in correcting it (with a soft pencil and minimal pressure, and without erasure), but not if the stamp has gum (mint or cancelled-to-order). Paper is more tolerant of graphite and clay than gum.

There are some additional issues where someone wrote catalogue numbers on the reverse. First, which catalogue was used to determine the catalogue number? Second, but less common, is a change in the catalogue number, like Scott did with Machins from Great Britain. (Scott changed the numbering from a regular catalogue number to catalogue numbers beginning with 'MH'.) And third, how about the previous owner misidentifying the stamp? Please take this as constructive information.

Thanks for all the replies. They were very helpful.