see the thing is, there arent chocolate polish in the USA. That is a common breed name used for dun polish, why in the world they wanted to do that is a mistery to me, but they named them that, that's why we also have khaki polish, cause they are really duns.
I have dun in my d'anvers, and their coloring looks exact to them, so I'd be willing to bet they are just dun based.
The breeding Philladelphia out lined for you is the best sure fire way to tell
As for making more of either, no that's easy, just breed him to what ever you want to put dun in and you will get more duns, that's the easy part. Hard part, also like they mentioned, seramas dont really breed true yet to any color or pattern, genetically they are a mess with that reguard, so you'll never know just what will come from them, but by using him as a breeder, you will get more dun patterned birds.
Yes seramas are one of the few US breeds with true chocolate in them, even still, dun is WAY more common. And to me, a better color to work with too, reasaon being you get 3 colors for the time and cost of 1 out of the breeding, black based, dun based, and khaki based. Kinda like Christmas when they hatch