Good information, Galeoptix. I take it you did this by enlarging scans and these were not photographed through a magnifier, correct?
Great if the silver coin works on the back as well. Looking farther, someone posted a comment on another website I found that the mark left by a silver coin was tarnish, but your example seems to disprove that theory.
As for the the St. Helena stamps I have, they are MNH, so it won't work on them.
As for the Steadtler erasers, I have used them for years. My mother drew schematics (drafting). While the Steadtler's are expensive, if you know what you are looking for it's possible to find similar plastic erasers at a dollar store. I picked up a three pack for a buck, a few years ago.
Here's what I had to say about erasers back in 2008 at the following thread.http://goscf.com/t/3268&SearchTerms=eraser
I never use the old style pink eraser, especially the
ones on the tops of pencils because they dry out and
can smudge. I only use old type cleaner that came in a
purple block and could be kneaded or the more recent
white plastic erasers, used in drafting, that will do
a very clean job. I erase a lot of things from covers
and haven't had a problem, yet. The type cleaner was
for cleaning the keys of the old-style inked ribbon
typewriters, so try to find some now. It seems to work
by pulling things into itself and leaves no crumbs.
Lately I've preferred the Steadtler Mars white plastic
eraser, and have listed a couple of links. It's used
widely by draftsman, so should also be available locally
at any art or drafting supply store. You might try it
by lightly marking up some envelopes as you might find
marked by a collector. I generally don't rub in both
directions, but only away from myself.http://cgi.ebay.com/2-Staedtler-Mar...aultDomain_0http://cgi.ebay.com/STAEDTLER-Mars-..._Accessories