Recently several beautiful and rare Kirby pencil photocopies turned up for auction. I was fortunate to acquired 16 of them from The Mighty Thor and The New Gods. Some of the best copies are from The Mighty Thor issues 156 and 157 featuring Mangog. These stories were originally published in September and October of 1968.
Back then Marvel would periodically make photocopies or stats of an issue and send them to the artist for reference. In some cases, copies were made at the beginning of the process when the pages were received by Marvel from the artist, who generally worked from home. In other cases, like these photocopies, copies were made right after they were lettered by the Marvel staff letterer, in this case Sam Rosen. Generally, fully inked copies weren't made because the inker usually worked remotely, so by the time fully inked pages were completed, the artist/penciler was already on the next issue or later.
When Jack Kirby drew an issue, he plotted the stories and put scripting notes in margin for Stan Lee to use for writing the dialog. You can see these notes in the margins of these photocopies. Kirby never saw the final dialog that, in many cases, was modified or scripted in a way he didn't intend for. So getting these copies was important so Kirby could move onto the next issue and plot with consistency.
While some artist might have trashed these reference copies, Kirby kept an extensive archive of them all, storing them in hundreds of manilla folders. Thanks to his efforts, today we can glimpse at pure Kirby pencils for enjoyment and research.
Click any photocopy thumbnail below to enlarge in a lightbox: