R 20th Century, the 15-year-old gallery that specializes in post-1945 international design, has stepped up to fill the void left by the recent closure of Moss. Founders Meyers and Snyderman have contributed to some of the field’s major recent museum exhibitions — MoMA’s “Century of the Child” among them — and the duo is also charging headlong into emerging markets, participating in Design Days Dubai and cornering the market on 20th-century Brazilian design.