Gretel head (newer version), interior, with eyepockets.
Gretel head (old version), interior close up, without eye pockets. (note: debris inside is residual eye putty after eye removal, not the mold.)
Gretel head, head cap partially removed.
Like most Obitsu large scale heads, the head cap is kept on by the rim of the head (the hole where the cap goes) fitting around a wider "lip" in the cap and into a groove. Removing and replacing the head cap involves some squeezing at the sides (above the ears) and prying the cap off or shoving it back in. Neither being very graceful, but effective and cheap. Kiki-chan has fitted a few Gretels with magnets for ease of entry. I haven't because I don't go in there much and when I do, being a big burly guy, I just manhandle them right off. Grrrrr!
As far as which is better, with or without eyepockets, it's really personal preference. The eye pockets can eliminate eye putty, hot glue, etc., and "fit" most 16mm eyes, but as all 16mm eyes aren't the same size, some fit better than others. For those difficult fits, a little snip or two with the scissors makes insertion easier. With some of the smaller 16mm eyes you may still need a little eye putty to keep them from rolling around.
The heads without eye pockets allow you to use larger eyes. In one of my Gretels I've got 19mm eyes. With a little beveling on the interior, they look great.