aHFo3
The front sight is what caught my eye. Looks very much to be a Beretta barrel, maybe for a Model 948B which had both a regular length 3.3" barrel as well as a 5.9" barrel. It was imported into the U.S. from 1948 to 1958. Some time later Beretta also offered a similar two barrel arrangement (3.5" and 5.9" barrels), with the Model 72.
It seems quite plausible then that your Grandfather sold or traded the gun but didn't include the extra barrel with it at the time.