Here is the FBI confirming Matthew Barton's dates for Stinson Trading and reflecting Isidor, Isidore, Isadore (Irving) Prosky as a founding partner. Mind the caps:

“On August 9, 1946 Special Agent WILLIAM O. McCUE advised that Confidential Informant [redacted] whose identity is known to the Bureau, stated that the S&P PHONOGRAPH RADIO SUPPLY COMPANY was owned by IRVING PROSKY, age 52, married, and HERBERT HARRIS, age 53, married, who were partners. The STINSON TRADING COMPANY was started by IRVING PROSKY and CHARLES R. STINSON in February, 1933, and was succeeded by the STINSON TRADING CORPORATION on April 18, 1936. This was dissolved in the latter part of 1937 when PROSKY and STINSON continued as partners until STINSON died March 24, 1938. Then the business was conducted as the S&P PHONOGRAPH RADIO SUPPLY COMPANY with PROSKY as manager. One MOE ASCH, formerly a partner, withdrew January 1, 1946, and established the ASCH RECORDING STUDIOS at 17 West 46th Street. HERBERT HARRIS was born in the United States and had been connected with the concessions business in 144 theaters. The S&P PHONOGRAPH RADIO SUPPLY COMPANY is the agent for manufacturers and jobbers of phonograph records selling to the trade and to individuals, and has an account at the AMALGAMATED BANK, Union Square, New York City.”
[FBI Silvermaster File, Pt.73, October 1946, NY 65-14603, p.82-85]
FBI Silvermaster File

Note: The file pages are a jumble and the above is just the Stinson stuff. See PDF page# 357.