e unit

[lionelsoni]

When I first put an e-unit into my MTH Big Boy, which lacks the clearance for vertical mounting, I put my own spring into the e-unit so that it could operate horizontally.  That actually worked just fine without any modification to the plunger; but, when I came across a purpose-built horizontal unit later, I swapped them for no good reason.

[cwburfle]

Your 1666 was not designed with an e-unit having a plunger return spring. Both engines should have a standard 3 position e-unit.
The factory e-units with a spring were used in the 1946 versions of the 2020, 671, and 726.
The e-unit would not have a switch. It was controlled by an arrangement of two jacks and a plug on the back of the motor brushplate.

If your e-unit does have a switch, then someone probably changed the e-unit plunger to the spring type (The end should be stepped to make room for the spring). The parts were available from Lionel.
IMHO, the change does no harm, and would not affect the locomotive's value.

 

[outlawc1]

I bought two 1666e engines and when I took the e units apart to clean them I noticed either one of them have a plunger return spring. Are they supposed to be that way both are prewar 2 position units .