This M-38 with a 106MM Recoilless rifle (dubbed the M40). Below is a short video, followed by a press photo that is available on eBay.
Press Photo from eBay:
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“1971 Press Photo US troops test jeep-mounted 196mm M-40 anti-tank rifle in MD. This is an original press photo. Guns – Artillery. Crewmen drive the new jeep-mounted 106mm recoilless rifle into firing position during recent tests at Aberdeen Proving Ground, Maryland, in this picture released Tuesday in Washington, D.C., Department of Defense. The battalion anti-tank weapon, designated the M-40, reportedly has greater range and more destructive power than any other weapon available at battalion level.Photo measures 8.5 x 7.75inches. Photo is dated 10-12-1971.”