From the picture you showed on the first post, the last stamp really seems ripped in half, but I really don't have the stamp in hand so I will have to believe you.
When looking for current value, I had sometimes gone to see Eric Jackson Revenue Stamps Web site (
http://www.ericjackson.com/). You might be able to find your stamps there. I check and there are C-275 and C-281, but none with SPECIMEN overprint. There is no C-304. I know that a SPECIMEN stamp won't have the same value than the stamp itself, but it is a place to start. You might even consider asking them for help.
Hope this might help.