(My edited comments)
We know in the 60s, at first Jack worked from Lee plots & Larry Lieber scripts, and eventually in late 1963 started adding margin notes. All I'm asking is has anyone seen examples of 50s originals where Jack's text is visible? I've seen 100s of 70s originals where Jack's text is visible underneath the letters. It's not usually completely erased. In fact, you can see it all over the artwork in the 1970s -- the letterer has to add a grid with a ruler and make the letters perfectly spaced -- Jack's original text was not, so much of it shows up on originals.
The theory that Kirby wrote text on the artwork in the 50s at DC is interesting, but is there any evidence? If not, I suspect Jack worked form a type-written script. And if Jack did submit a story, I also suspect he submitted a type-written script. In the 1950s at DC I 'd guess it would be easier for an editor to look at a type-written script and approve that script (plus make changes) than to try and decipher Jack's handwritten text on the original artwork, which would then have to be deciphered again by the letterer. Another reason why Mike Royer was such a professional -- I don't even know if Mike got extra pay for adding letters on Jack's 70s stuff.
So while intriguing to think of Jack working Marvel Method in the 50s (drawing stories and adding text) at DC, I honestly don't know of any evidence that suggests this was the case. But I'm not an expert on that material.
(End of my comments)
I went ahead and looked at the very first 1950s scan I have in my files. Here's a page from Alarming Tales # 4 pg. 5 (1958), from the Heritage Auction website.
You can see text in this thought balloon, but it's hard to tell if this is Jack's handwriting.
In this bubble the word "The" and "Rocket" looks like Jack's handwriting to me, so it does appear in the 1950s Jack was adding text to the original artwork. This book was published by Harvey so Jack may have been writing and editing the material, but I don't know if he worked using the same method at DC in the 1950s, although it certainly is possible -- a close analysis of any surviving Kirby 1950s DC originals might give us some clues as to who wrote those stories.