Here I have two coolantpumps, what is the difference?

I know, one has a hose attached to it but I mean the impeller. The steel one I found and recognised it as I bought it a year ago, the bronze came with another block and accidently I fitted it on my refurbished block. In my perception the steel impeller produces more flow and swapped it to my engine, together with a new thermostat.
Anyone recognises the bronze impeller as being an older version maybe?
Tony.

I would say that by looking at the impellers that one would give better circulation than the other or maybe one is a patent part ?