I haven't looked at that grand old kit for years, but I think the oval in question may be the remains of the hole for the switch that controlled the electric motor in the motorized version. (The kit first appeared in 1958; the motorized version was issued in 1961. My source is, as usual, Thomas Graham's
Remembering Revell Model Kits.) Revell probably modified the original mold for the deck, then modified it again to plug the hole. But the outline of it remained.
Revell did that sort of thing more than once. The latest Revell Germany reissue of the old harbor tug
Long Beach is marred by an oval like Wally501 described - plus some big, ugly lugs in the superstructure sides, next to the deck. The lugs originally were locators for a pair of screws in the motorized version; the whole superstructure was removable, for access to the batteries.
Maybe the worst compromise in the name of motorization, though, was what Revell did to its pretty Mississippi Riverboat, the
Robert E. Lee. When they motorized that one they not only removed the scale boilers (to make room for the batteries); they also made the hull about twice as deep as the original, more-or-less-to-scale version.
In all these cases the original kits reappeared minus motors later - but evidence of the motors, batteries, switches, and other motorizing components stayed. Too bad. I wonder if the pre-motorized versions (e.g., a
Robert E. Lee with the original, shallow hull) are worth more on the collectors' market than the ones with motorization scars.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.