15¢ Webster I.d. #163 Or #152? - Edited-...

I just received this in the box today and it was sold to me as a #163 but I'm not positive. From what I was able to find it could either be #152 or #163 (or #141 if it had a grill)
I'm trying very hard to learn all that I can about the secret marks and know that this particular denomination has never actually had a die-hard or set-in-stone "secret mark" designated to it (Although Scott catalog published it did back in 1934 I believe I found from my research) and so the upper right "coffer" has an anomaly that shows up on the different types of papers used by the different printing companies (National & Continental) and wear on the dies and is probably better suited for I.D.-ing that than the actual Scott number...I think I understood that correctly?
Any ideas on what this is? I'll do a larger scan of the upper left hand "coffer" for your scrutiny.

EDIT
My copy does not seem to have a grill either. I thought there was
maybe a trace of one but I was mistaken.
OK, here is my stamp:



Some of the informational sites that I used.

http://www.jamesdire.net/sm15bn.html

http://www.uspcs.org/uspcsBN.html

http://www.uspcs.org/NATLvsCONT-BNL.jpg

Here is the"coffer" or upper left hand triangle on my copy



Here is the "FIFTEEN CENT" banner on bottom of my copy:





Thank you for any help!

These have always been frustrating stamps for me, Jeff.

The heavier shading in the upper left triangle pretty much eliminates this from being a 152 (National printing), leaving two choices: 163 (Continental printing on white, wove paper) or 189 (American printing on soft, porous paper).

Scott also states that the "secret mark" can be found on two printings (American & Continental) but not all of these printings have this mark.

Your stamp has the mark so now all that's left to do is to identify the paper type, and you'll know what you have.

What I'd like to know is how do you decipher the difference between a 152 (National printing) and a 163 (Continental printing) if the 163 is missing the secret mark?

Brian

Hmmm... It's not wove paper or at least it doesn't have the diamond like "grid pattern" so hard white maybe? I'll have to read up and take a closer look at the paper and see if I can dial it on a bit better. Thank you?