I have a 17.5" T6 cob 3D on my pony, It's a nice saddle very difficult to tell from leather unless up really close...I show locally with mine with no problems.
It fits my boy ok, but he is not particularly cobby/flat backed, I think it would fit a flatter backed horse better but saying that it was one of the few that did fit mine. I find when he's full weight it fits really well it's just when he drops a little too much it doesn't.
The interchangeable gullets are great though as is the fish as without these I would have had to have bought three different saddle in the last year due to his improving muscle wastage and changing of shape.
Harvey is a 13.3hh welsh cob type