Took a part several cake/flakes/slices of MR that I got in an 8oz tin and separated every different type of tobacco.
There is the red mixed with an oriental, and then peppered with a little perique and a little more latakia.
I smoked the red component with the stoved VAs taken out and without as much of the latakia as I could take out, and on first light I get the sour taste i was referring to, though also the red cake taste. It's cased/topped with something for sure. Not able to detect what it is yet, but if this tobacco is being made by McClelland, and I have every reason to believe it is (given the tobaccos in it and the fact that SPC's Seattle Evening is blended by McClelland), then I'm thinking the casing is similar to that in McClelland's 5115 blend. I've ordered some of it and should have it this upcoming week to try.
What makes MR unique and appealing, to me, is that sour taste you get immediately and throughout the majority of the bowl.