one thing i seem to be noticing is that the detail on the 'electric' version looks extremely sharp (pilot, steps, riveted bands); perhaps an indication of an early piece out of the mold. add to that the single attached dome whereas other 994/995 shells had either two or none (embossed as are all the MMM clockwork locomotives i've ever seen). finally the lack of Mickey on the smokebox front is certainly different.
my guess would be that this is a pre-production model. the set cost may have had something to do with the final production decision. going with clockwork would keep the price down and might also be a factor with the intended audience (ie; kids). embossed domes vs. attached pieces not only kept the part count lower, but would not require the additional assembly step. Mickey's smiling face may have still be in the works when this piece was made.
bottom line with Marx, it's hard to prove something didn't exist, but this is the first MMM locomotive/ shell i've ever seen without a slot for the clockwork motor brake or with the single attached dome configuration.
nice find.
cheers...gary