Formal vs Real Subsumption is one of the best sections I've ever read by Marx. Here in MECW 34, it gives some cool drafts of that chapter, too, and some crucial info on the divergences that were Theories of Surplus Value and Herr Vogt in his respective economic studies (relatively respective to the drafting of A Contribution Towards Political Economy and Capital draft 2). Thankfully, somehow, we got only this chunk (secret 'Chapter 6') of draft 3 of Capital! Also, David Harvey's Limits of Capital p.106-110 is great source for connecting Formal and Real Subsumption to the relevant parts of the published Capital, Vol.1, where everything was reorganized, obviously,