A couple things:
First, I pretty much agree with Jim and Scot's comments.
Second, be careful with population reports. There are two Merkle subjects in the T206 Set: Merkle Throwing and Merkle Portrait. For T206 Old Mill, yes SGC shows a pop of 7 for Merkle Throwing. But it also shows a pop of 6 for Merkle with no subject description, with the highest graded of these being an SGC 80. For other backs, it does show some being Merkle Portrait. So if these Merkle No Description were really Merkle Portrait, wouldn't they most likely be listed as Merkle Portrait?
Merkle Portrait could possibly be found with an Old Mill back, but it has not been confirmed on either
T206Resource or
Ted Z's Old Mill Backs Survey. PSA does show a pop of 2 Merkle Portrait with an Old Mill back, compared to a pop of 7 Merkle Throwing with an Old Mill back, but this may be an error. So I'd say most, if not all of the SGC Merkle No Description are actually Merkle Throwing.
This is not a knock on Wazoo, as he does state that SGC is limited. Nor is it a knock on SGC. Rather it is a caution on interpreting pop reports. PSA offers a similar conundrum in that of the population of nearly 150,000 T206, about 63,000 are listed as "Unknown Back".
So on one hand, a portion of the SGC T206 population has a known back, but no description for the subject. And on the other hand, PSA has a subject description, but the back is unknown.
Third, I am finding it possible to cross-reference between SGC and PSA pop reports, and along with data from the wild (ebay, auctions, BST, etc), knowledge of which subject/back combos are possible, and statistical methods, to begin modelling an estimation of the T206 backs population distribution. I've been at it for quite a while, and it will probably be quite a while still until I'm ready to discuss what I'm doing and present results, but I will throw out the attached graph as a "teaser". (caveat: Further research will most likely result in a change in the order across the horizontal axis (such as the American Beauty backs probably shifting towards the right) as well as the relative population distribution curve. But it does demonstrate what I am working towards.)
Best Regards and Happy Collecting
Craig