For a few years now, IMDb.com staff have refused to add "uncredited" roles to Stallone's filmography when I asked them to do so, although they briefly added some of them before. And, it seems it was I who actually discovered him in about 4-5 films before anyone else.
Anyway, when Stallone was starting out in the late 60's in the industry, he struggled to find work, being deemed too weird. Not talented. Etc. He kept at it, going to tons of castings, but mainly only achieved small roles as an extra, including a tiny but memorable part as a mugger in a subway, annoying Woody Allen in Bananas.
I once found a screenshot taken from a September 1970 theater flier that said he was in Downhill Racer. It omitted info about where he is in it. I then watched the film. At first, I couldn't find him anywhere, but then I saw him in a restaurant sitting at a table. The film stars Robert Redford and this super attractive Swedish woman, called Camilla Sparv. However, I got no gratitude from his fans. All I got was 3627 idiots saying to me when I put a clip of that scene on YouTube, that it wasn't even him...but it was him. He was only like 22, but it was certainly him. According to some sources, he worked in Switzerland as a teacher around that time, and that film is partially set there.
I once read stuff about Elliott Gould meeting Stallone and the guy who played his old manager in the first few Rocky films. Stallone said to him that he never admitted to being an extra in Hollywood films (but why?) and he said, however, that he was in MASH. Although Gould didn't say where Stallone was. I had a feeling it was probably an outdoors scene. Then I finally think I found him, based on what little clues I had. There's a scene near the beginning where Donald Sutherland was sitting at a table. It's a table with a hood over it, because it's a comedy about the army, and this was like a ranch where troops had their meals out in Malibu. There's someone called Radar who overhears him saying something. You hear this Radar guy saying, "Well - I'm the commanding officer and I'm gonna get to the bottom of this..." and he quickly walks past a few people to ask him about a stolen jeep, which he denies stealing. I'm sure Stallone is the guy with the beret in that scene, seen for only a second as Radar approaches Donald Sutherland, and Tom Skerritt.
I saw a not too old interview where Stallone said he seen Ryan O'Neal boxing in 1971 as he did extras work. He didn't mention any movies at all. So I did a bit of checking. There's a film Ryan O'Neal was in with Barbra Streisand. What's Up, Doc? (a comedy) set in a hotel. I saw the filming dates were like late 1971 (despite the film being out in early 1972). So I paid close attention to scenes with extras darted about, and they were all pretty blurry looking, though. However, there's a part where Barbra Streisand grabs a piece of carrot as she's talking on the phone. Away over to the right, I briefly seen Stallone and a woman (maybe his first wife, Sasha) walking past. He was only on screen for like 3 seconds. You could still kind of tell it was him, because of the black hair. I've put these films on Wikipedia, but I don't have any citations.
To be honest, I wish Stallone would just publish a book already saying where he is in what X, Y and Z movie. Is he maybe embarrassed about this? I'm not...