The Reduco Corporation
The Reduco Corporation is a fictional weight-loss company with offices in New York City and Boston.
According to an in-depth article on the production of Rope (1948) by American Cinematographer editor George E. Turner, the red neon sign which forms the director's cameo in that film was intended to be the a follow-on gag and was another advertisement for the Reduco Corporation. The film is set in a New York apartment, where the company has an office.
As the sky darkens, lights begin appearing in the windows of the buildings and neon signs flash on. One of these signs depicts before‑and‑after figures advertising a product called Reduco. The model for this was the director, whose portly figure is well known, making the token personal appearance that had been a tradition in his films since The Lodger, made in 1926. He had used the Reduco gag as a newspaper ad in Lifeboat (1943), another one‑set show in which he could not logically do a walk‑on.
This was also confirmed by a brief comment made by Hitchcock in his 1948 article, "My Most Exciting Picture":