Snyder's third nostalgia trip to Ike and Mama's cozy multi-ethnic South Bronx starts with Ikey and friends helping other friends sneak into the Saturday afternoon two-for-a-nickel movie. They're caught, but not before the newsreel which tells of D. W. Griffith filming Way Down East at his Mamaroneck estate north of the Bronx. AS extras will be paid an unimaginable $1.50 a day, the boys set out for Mamaroneck on Sunday, hoping the pay will satisfy Mama's directive to pay back the theater owner, with enough left over to take their families to the movie they'll appear in. It's a long bike ride, slowed at the end by snow, and at first it seems that all the boys will get from Griffith is a nickel apiece for shoveling. But when the camera starts shooting their spontaneous snowball fight it all turns out as planned. . . right down to the surprised families watching Way Down East at the outdoor movie in July. ""America gonif,"" says Mama. ""Only in America could such a thing happen."" Probably only in Snyder's South Bronx --where a little innocent American dreaming mixes easily with her chicken-soup humor.