How to tell a Koester Bread from an E121 or W575?

[dougscats]

I ask this question only having learned about Koester Bread's last week, when I auctioned off an Ed Brown.

I have one card left from that lot, a Waite Hoyt, also with the same scrapbook/letter back that the Brown had, and I wonder if it's a Koester's.
I tried to do my homework this time, and Google brought me back to Net54:

http://www.network54.com/Forum/15365...read+Babe+Ruth.

In this post, ellyn2005 maintains that the thickness of the card and the quality of the picture distinguish the Koesters from the W-575:

"... [Regarding] the differences between W575-1's, E121's and the 1921 Koester's Bread.

W575-1's..These cards have thinner stock, are grayer and do not have the clarity that E121's
or Koester's Bread [do] ... We do acknowledge that this 1921 Koester's Set is a similar issue to both,
kind of a hybrid but there are distinctive differences..

This subset [Koester's] had blank backs, similar to W575-1 but card stock is similar to E121..."

So, given the scan below, can you tell if this is a Koester's Bread?

It is thick, and the picture quality is pretty good, so, if ellyn2005 is right, I wouldn't think it's a W575.
I guess the only way I could tell whether or not it's an E121 would be to take the scrapbook/letter off the back.
But I hesitate to do that, being an amateur, thinking of using a handkerchief and a steam iron.

So, I put it to you, for your expertise and advice.

If it's a standard E121, I'm keeping it [in my eclectic hall of fame collection].
But should it turn out to be a Koester's, and presumably valuable, I'll most likely auction it right here, as good fortune would have it.

... Or maybe trade it for some T206's.

So, what do you think: