Looks to me like there's a hole on the rim, right side, in the Science Museum photo. Perhaps that's where the suction devise attached to the cup.
To me the description reads as if breast reliever and breast pumps are synonymous. "Mothers used breast pumps, or relievers, to remove their milk. " If there were no commas in this sentence, the implication would be that pumps are distinct from relievers, but with the commas it says they're the same. I think. In American anyway; perhaps British English is different?
I had that same thought - that description was written by a man!
I suppose originally the spout arched on Paul's, with a removable hand pump on the end, like a curved turkey baster.
How much does it hold?