People say "they were making toys."
The weird 156 is this one:
156a1 Dark green, type 2 motor, black fiber plate with manufacturing plate, diecast wheels with simulated spokes, riveted couplers, pedestal headlight, holes in diecast wheels, no weights on pony trucks.
I've never seen a 703 with gold vents, but here's a 156 with them. With the exception of the pony wheels and the gold vents, this is the same as my 703. The number and the wheels means it was made later than the 703. The number can't be changed.
I could come up with a few different guesses about why this was made, but all would just be guesses. The only fact I can say is this was made AFTER the 703 locos. Date wise, I think it's no earlier than 1917 when the number change was made.
Here's a fun fact - Lionel made trains in Newark from 1913-16. In 1916 they moved to Irvington. 1916 is one of the years in standard gauge where a lot of odd items were made. Coincidence?
For the later mid 20s oddities like the 156X above, Lionel kept portions of Newark plant for use as storage into the mid 1920s when the Irvington plant was expanded. Coincidence?