This may have been discussed before but I am new to this Forum.
In the 13/14 era, side and tail-lamps were black(steel)& brass, with brass chimney caps and top surfaces of chimney tiers & steel to the balance. I have some made in reverse with steel chimney caps very clearly stamped as Ford lamps by Brown. Also a tail lamp completly in steel.
I'm wondering how common these lamps might be. I assume made on a friday or night shift ?
They were made both ways. If your lamp has a steel top, then the slanted curved portions will most likely be brass.
To answer your question it hurts the value of your lamps if they don't have brass tops. The repro part sellers have new tops for some brands, and they are easy to swap.
What you are seeing is pretty common.