I'm surprised you've never heard of the SU-2: 53 aircraft ordered in 1933 [A9077, A9079/9108, A9110/9121]. Your photo depicts aircraft #9115. The only odd thing is the "S.U.T.-2" designation in the caption, presumably a typo.
At the same time the O3U-3s were ordered, 65 O3U-4s were ordered to be powered by the larger hornet engine. All of these aircraft were designated SU-2 or SU-3 since they were delivered to the Marines. Forty-three became SU-2s and 20 became SU-3s. The suffix number change indicated minor equipment changes. One became the XSU-4, which differed from the others in cowling, cockpit and fin. The SU-2s and SU-3s replaced the SU-1s aboard the USS Lexington and USS Saratoga. They served in various utility roles with Navy units. Some were later made into SU-4s. As late as 1941 some of these aircraft were still stationed at various Navy Air Stations.
Source: http://www.voughtaircraft.com/heritage/products/html/o3u-4.html