On May 13, 1997, at 1215 central daylight time, a Piper PA-12, N712MW, flown by a private pilot experienced a loss of directional control and a collapse of the left main gear while landing on runway 03 (5,399' x 100') at the Sheboygan County Memorial Airport, Sheboygan, Wisconsin. The pilot and passenger were not injured. The airplane was substantially damaged. Visual meteorological conditions prevailed and no flight plan was filed for the 14 CFR Part 91 flight. The local flight originated at approximately 1115 cdt. The pilot reported that prior to takeoff he checked the weather and the winds were reported as calm. He reported they took off and departed the area for a local flight. Upon returning to the airport he called the Unicom and was informed that runway 03 was in use. He does not recall what the wind conditions were reported as. He did report, however, that while entering the traffic pattern on downwind he checked the windsock which was "hanging straight down." On final approach he once again checked the windsock which was still indicating a calm wind. He reported he made an eventful three point landing without having to apply any control inputs for wind correction. He reported that during the landing roll "... the right wing came up quite rapidly. I applied right aileron in an effort to bring the wing down. At this point the tail also started coming around real fast turning us to the right." He reported he was able to bring the wing back down, but he lost directional control and the airplane slid sideways coming to a stop on the right side of the runway. The left main landing gear collapsed during the accident. The pilot reported that after the accident he noted the wind sock would quickly change from full extension to limp. He reported he checked the wind conditions once in the lobby of the airport and they were reported as 100 degrees at 7 knots. Winds reported at Oshkosh, Wisconsin, 38 miles northwest of the accident site, at 1029 cdt were calm. The winds reported at 1253 cdt were from 090 degrees at 8 knots.