Heckscher Ballfields sign. Technically the ballfields are part of Heckscher Playground. The 1935 plan[1] for this area says:

The [current] ball field, which occupies about one half of the area of the playground, has always been a bare expanse of dust and cinders with no orderly layout. For days after every heavy rainstorm in the Summer this field is nothing but a mud hole. It will now be regraded, properly drained, and seeded. There will be four soft ball diamonds and two hard ball diamonds, which will be permanently and properly located for the most efficient use of the area for ball play|ng. This field will be laid out in such a way that it can be used for football by the smaller boys and for field hockey by the girls, during the seasons when baseball is not played.

References:

1. NYC Parks Department press release, March 24, 1935, plans for Heckscher Playground renovation.
2. NYC Parks Department press release, December 30,1936, opening of renovated Heckscher Playground.
3. New Deal Assistance in NYC Parks Department Projects, 1934-43.