Here's the answer to your question on this thread concerning what you quote from the Grundisse.
In part you quote:
"...It would therefore be unfeasible and wrong to let the economic categories follow one another in the same sequence as that in which they were historically decisive. Their sequence is determined, rather, by their relation to one another in modern bourgeois society, which is precisely the opposite of that which seems to be their natural order or which corresponds to historical development. The point is not the historic position of the economic relations in the succession of different forms of society."
^^^^^ CB: When Marx says "economic categories... were historically decisive"
he implies necessarily that what Engels refers to as a "simple" historical stage of commodity production was actual, it existed historically and not only existed but was in some sense historically decisive. So, Marx agrees with Engels that his discussion of exchange without money existed historically and is not just a logical category. Afterall, Marx out and out says said economic categories were historically decisive and were in "historical development" in the quote you brought to the thread.
Furthermore, the first Chapter of _Capital_ Vol. I does follow the Grundisse quotes' order. The first words are that in the societies in which the capitalist mode of production prevails wealth present itself as an immense accumulation of commodities, or similar words. Well, right there the text starts in reverse historical order. Capitalism comes last historically, but is presented first in the text. Then in the text the commodity is analyzed more in its fullest and capitalist form in the next several pages. The analysis of the earlier historical form of less than fullblown money doesn't occur until a ways on in the text.
So, there it is; as the Grundisse would have it, reverse historical order: capitalist forms or categories and then the pre-capitalist forms/categories. But the pre-capitalist forms do correspond to historical, or formerly but actually existing pre-capitalist forms as Engels claims.
Engels' choice of "simple commodity production" we might trust because his English and German were reputedly very good, and he had extensive personal discussions with Marx on so many subjects, it is hard to believe they didn't communicate on the issue at dispute on this thread, it being very important in the book.
Here's another statement by Marx on point:
Every beginning is difficult, holds in all sciences. To understand the first chapter, especially the section that contains the analysis of commodities, will, therefore, present the greatest difficulty. That which concerns more especially the analysis of the substance of value and the magnitude of value, I have, as much as it was possible, popularised. [1] The value-form, whose fully developed shape is the money-form, is very elementary and simple. Nevertheless, the human mind has for more than 2,000 years sought in vain to get to the bottom of it all, whilst on the other hand, to the successful analysis of much more composite and complex forms, there has been at least an approximation. Why? Because the body, as an organic whole, is more easy of study than are the cells of that body. In the analysis of economic forms, moreover, neither microscopes nor chemical reagents are of use. The force of abstraction must replace both. But in bourgeois society, the commodity-form of the product of labour — or value-form of the commodity — is the economic cell-form. To the superficial observer, the analysis of these forms seems to turn upon minutiae. It does in fact deal with minutiae, but they are of the same order as those dealt with in microscopic anatomy.
With the exception of the section of value-form, therefore, this volume cannot stand accused on the score of difficulty. I presuppose, of course, a reader who is willing to learn something new and therefore to think for himself.
http://www.marxists.org/archive/marx/works/1867-c1/p1.htm
^^^^^ CB: This pretty clearly implies that Marx thinks that the less-than-money-form value-form existed before capitalism back 2000 years.
You can ask me as I asked you, "so what ?" What does the fact that simple commodity production, the law of value, existed historically as Engels claims matter for theory of capital today ?