Thanks to Jack's grandson Jeremy Kirby for sending me a high quality scan of the Marvelmania piece we looked at recently. Let's zoom in and take a closer look at three examples from the original artwork.
Maybe one of the last times Jack drew Iron Man. Probably the only time he inked the character. Notice Jack penciled the outline of the image, inking those main contour lines with a pen, then filled in the inside details of the image with a brush.
If I didn't know better, I'd swear John Romita drew this image of the Rhino, but it appears Jack wanted to include a variety characters, so he probably swiped this image from a Romita Spider-man issue.
It's actually a little sloppy -- the shadow under the face doesn't have the same light source as the shadow behind the right hand. Maybe Jack's technique would have been more effective if he had penciled out the whole image and added inks, instead of penciling just the outline then adding the details only using a brush. Plus this representation of the Rhino suggests to me Jack wasn't at his strongest when copying the style of another artist.
Finally an image of Jack's Dr. Doom.
Again, it appears Jack added all the interior brushwork without any pencils to guide him -- a pretty common practice artists who are inking their own work use to save time.
You can see the brushwork is not nearly as dark as the pen lines and looser.
Jack probably would have been able to ink his work in half the time it would have taken another inker by filling in details like the metallic sheen of the armor freestyle, using a brush.
At the same time, these images are a little sloppy, and lack the famous thick contour lines/thin interior line style inkers like Chic Stone and Joe Sinnott made famous on Jack's work at Marvel during the 1960s, so maybe this was a preliminary piece Jack did to get back into the groove of inking his own work. When he worked on the larger Marvelmania pin-ups he decided to use a more traditional approach -- detailed pencils; thin-to-thick lines with a brush for outlines; and a pen for interior details.
