Does anyone have a side by side photo of the metallic rifle oiler used for the M1903 Springfield in WW1 vs the rifle oilers that were used in WW2 ? 

Were there any brass ones or were they all brass plated nickel ?

I know that by WW2 they had a rounded end to help unscrew the oil section. Any other differences ? 

Two on the right are the later version of the oilers, note the different checkering on the sides of the oiler caps.

 

 

Also, they are nickel-coated brass (not the other way around).

Both were used regardless of pattern into WW2 when the “plastic” ones were deemed acceptable and freed up brass for other uses.  Not like they stopped issuing one.