I thought these were only used on the M-38A1s. I’m surprised they were testing this on an M-38 as late as 1954. But, perhaps that was just the photo’s release date and not the date of the photo.
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“1954 Press Photo US soldiers fire rifle on a jeep at Aberdeen Proving Ground, MD. This is an original press photo. Guns – Artillery. Aberdeen Proving Ground, Maryland. The Army’s new battalion (BAT) 106 MM. recoilless rifle is mounted on a jeep and fired during a demonstration at the Army’s Aberdeen Proving Grounds. The rifle has greater range and destructive power than any other weapon now available on the battalion level.Photo measures 10 x 8.25inches. Photo is dated 10-12-1954.”