caper already mentioned stampsmarter, which is a great resource.
There is also the 1c plating archive, which contains a very accurate plating record.http://www.slingshotvenus.com/Frank...hv_Main.html
Finally, on this forum, if you search under a thread subject of:
"1c 1851" or "1c 1857" that will reveal most of the by-plate discussion threads that we've had here.
I use the bookmark facility of this forum to effectively go back and forth among them. Its one of the top buttons on the page, under "tools". I bookmark via this forum all of the 1c (and other) pertinent threads I want to remember. That saves later searching.