And since Eisman is the main artist, when there are figures (of secondary characters) in another artist's style, I'm going to say that artist was assisting Eisman unknown to the Harvey editors and thus a ghost. The artist I see is Gus Lemoine from Archie (most notably Madhouse). These pages are from "O. O. Heaven" and "The Playpen Contest."
Upon my first look I went along with the GCD, wondering if there were unknown artists on a few early stories, but I think it's the inconsistency in inking styles among different stories before one style is settled upon with issue #3. (Eisman himself or other inkers? I can't tell.) I'd give the main pencilling credit on all "Bunny family" stories to Eisman.
Bunny pencil assists by Gus Lemoine
|Dec/66||1||Rookie Secret Agent O. O. Heaven|
|The Mush Meal Caper|
|The Playpen Contest|
|Apr/67||2||The Ultrazaric Decodifier|