The original recording of "When Mussolini Laid His Pistol Down" by Bob McCarthy aka Merle Travis is hard to find on the web, but Denver Darling's cover version (recorded December 10, 1943) is available here*, even with a transcription of the novelty lyrics written by King label owner Syd (then Sid) Nathan and Roy Starkey, who was a WLW Radio, Cincinnati, performer like Merle Travis at the time. As said here**, "Early recordings on King came almost exclusively from current or former WLW performers."
The melody, however, was apparently taken from "Pistol Packin' Mama" by Al Dexter who stays uncredited here.
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* Authentic History Center: WWII In American Music: Axis & Allies
** Industrial Strength Bluegrass: Southwestern Ohio's Musical Legacy, edited by Fred Bartenstein and Curtis W. Ellison, Jan 2021.
FANtastic that you got this image added!! I have a maybe 200 King's but I haven't started checking them off yet - maybe I'll get lucky and have one to add to the list, but I doubt it based on the completeness of your list!
Label master (matrix) information added to the notes as given in JEMF Quarterly 1968, p. 66 (June, No. 10). As the addenda in JEMF Quarterly 1969, p. 111 (Autumn, No. 15) confirms, both King 500 and 501 were actually released (in contrast to the 1968 text, saying "were never released").
The second King record, made on the same session as the first, and the recording debut of Merle Travis as solo artist (under pseudonym because he was under contract to WLW Radio). Issued in a small number of about 100 copies, with very poor sound quality. See King 503 for more.
Although possibly written by Merle Travis, the songs were apparently credited to King owner, Syd Nathan. "When Mussolini Laid His Pistol Down" was actually covered by Denver Darling in Dec 1943, credited to "Roy Starkey-Sid Nathan" then, Decca 6094 B.